Author 


Title 



Vft- 

. 30 - _ 

.AUg. _ 

»<^4:3c3u. 


Imprint 


1«—47372-2 OPO 














I 








I 


I 


% 


I 


f 





t 




\ 




I 




V 





» A 

" y 








I 


i 


t 




3 - SEP - & 


u 


FREE GUNNERY 
INSTRUCTORS’ T^SNING 
MANUAL 


. rreyrvrt 



DBeDoaeBsiEi) 
CfSHARY oe CONOM0S 

FAC. Fite No. 


flU8 261957 

AUTHOBUJla.*^/'^. lU. S. Aii VV 




TRAINING DIVISION 
BUREAU OF AERONAUTICS 
UNITED STATES NAVY 











'tl ^ 


~f~ - 




FREE GUNNERY 
INSTRUCTORS’ TRAINING 
MANUAL 



Published By 

TRAINING DIVISION 
BUREAU OF AERONAUTICS 
NAVY DEPARTMENT 
WASHINGTON, D. G. 





, , 

iq43"' 


FREE GUNNERY INSTRUCTORS’ TRAINING MANUAL 


This manual is a revision of the one compiled and used for the 
first four classes of Free Gunnery Instructors at the Naval Air 
Station, Pensacola, Fla. It is more complete and the subjects are 
more clearly covered. Advantage has been taken of all reliable 
sources of information though, unfortunately, they were only too 
few. The information contained in this manual is sufficient to enable 
any gunnery officer to train his free gunners effectively. Old ideas 
must be completely discarded and these new methods of fire control 
adopted. 

While other officers have contributed toward the production of 
this manual, special credit is due Lt. Comdr. James G. Lang, U. S. N., 
and Lt. D. C. Higgins, A-V (N), U. S. N. R., for a major part in 
the compilation. Lt. H. B. Jackson, A-V (P), U. S. N. R., con¬ 
tributed largely to editing and assembly of the material. 

Training Dr ision. 

Bureau or Aeronautics, 

Navy Department, 

Washing ton ., D. G, 


(ii) 


TRA^ISFE!I 

59 

NOV 8 1945 

Serial Record Division 
The Library of Congrew 
Copy. 



CONTENTS 


Section Page 

Preface_ v 

I. Mathematics essential to free gunnery_ 1 

II. Introduction_ 3 

III. Construction of the 35-mil sight__ 4 

IV. Range estimation__ 8 

V. Theory of free gun sighting_ 13 

VI. Apparent speed sighting_ 21 

VII. Tracer__ 32 

VIII. The machine gun_ . _ 41 

IX. Ammunition_ 53 

X. Bullet pattern_ 63 

XL Boresighting and harmonization_ 67 

XII. Ballistics_ 76 

XIII. Optical sights_ 88 

XIV. Turrets_ 93 

XV. Recognition_ 107 

XVI. Safety precautions_ 110 

XVII. Glossary_ 112 

XVIII. Synthetic training devices- 115 


(III) 





















PREFACE TO MACHINE GUN INSTRUCTORS 

In order that the high quality of free machine gunners so essential 
to the safety of Naval aircraft be obtained, it is necessary that they 
receive the best possible training. They must have instilled in them 
the highest degree of pride and enthusiasm. They must realize that 
without their services the primary mission of aviation will fail. It 
must be pounded into them that the history of the present aerial war 
proves that without the rear gunner aviation cannot produce results. 
Obviously, the single-seater type of plane has no need of a rear gunner. 
Its protection is provided by its offensive power, speed, and maneuver¬ 
ability. On the other hand, it has been only too clearly shown that 
the fighter plane wins no wars. The war of today will be won by the 
bombing plane or one of its variants, all of which are multi place. 

These larger planes have important missions to perform, among 
these being patrol, antisubmarine work, and bombing. If the pilots 
of these planes are to accomplish their mission, their minds must be 
easy and free from doubt regarding possible attack from astern. On 
a bombing run, for example, the pilot’s entire attention must be 
devoted to his instruments if accuracy is to be obtained. This can 
be accomplished only when his rear gunner is a tower of strength. 
The gunner must be reliable, and his gunfire must be deadly. His 
sight must be excellent, and, above all, he must be alert and able to 
use his gun under any conditions. He must have confidence in his own 
ability to shoot down the enemy who dives on his tail. He must 
be cool in emergencies, and, above all, he must hit and hit often. This 
means he must know when and how to shoot. For this a thorough 
knowledge of recognition, range estimation, and sighting is essential. 
Only in this way can aviation perform its primary mission which is 
that of gaining control of the air. 

The fire from a free machine gun or turret is only effective when 
it is accurately aimed. The number of rounds fired in a burst from 
one or two machine guns is so small that the spray effectiveness in an 
engagement is almost nil. Only the poorly trained gunner will attempt 
to handle the bullet stream as a hose pipe (that is, swing his gun back 
and forth in the general direction of the enemy) and hope for a hit. 

Flying a plane has its attractive features, to be sure, but the pilot is 
useful only in proportion to the collective abilities of his plane crew. 
A poor radioman can defeat his mission, an unreliable mechanic will 
permit the engines to falter, and a poor machine gunner will permit 
his plane to be shot down. The mission of the plane fails because of 


(V) 


VI 


its weakest link. Thus, the free gunner is as important in our scheme 
of things as any other member of the plane crew. One might go 
further and say that he is the most important man in the air. 

The free gunner must be taught to know and appreciate these 
things. He must be made to feel deeply the responsibility of his 
job. He must be saturated with the desire to scan the skies constantly 
and to shoot straight and hard; he must be imbued with the spirit that 
he is one of the team, the success of which is dependent upon each and 
every one. He must be made to feel that without his services his pilot 
is helpless and that the success of aviation hinges upon his accuracy of 
fire. He must be imbued with the spirit that a failure on his part will 
result in failure to reach objectives, in plane casualties, with the pos¬ 
sible loss of battle. In short, the free gunner must come to feel 
that he and he alone is responsible for the safety of the plane and for* 
the successful accomplishment of its mission. 

The general sighting methods described in this manual have been 
tried and proven in combat. These methods and the logic behind them 
should be studied and understood so thoroughly that the gunner’s reac¬ 
tions in the air will be almost automatic. A thorough mathematical 
understanding of free gunnery is unnecessary for good shooting, but 
the gunner should understand the logical reasons for making sighting 
corrections and realize the limitations of his gun or guns. He should 
realize that tracer is not a cure-all, but a valuable aid in sighting when 
used correctly. 

The training of free machine gunners is of the utmost importance 
and one around which aviation and the Navy can succeed or fail. 


Section I. MATHEMATICS ESSENTIAL TO THE STUDY OF 
FREE GUNNERY 

The mathematics required for an elementary understanding of the 
theory of aerial free gunnery extends no further than (1) the 
theorem of similar triangles; (2) the theorem of Pythagoras; 
(3) the solution of problems in proportion, and (4) a knowledge 



Figure 1.— Similar triangles. 


of the relationship between the sides of 30-60-90 and 45-45-90 
triangles. All this can be recalled from high-school plane geometry. 

1. Similar triangles ,—Similar triangles are triangles whose corre¬ 
sponding angles are equal, and whose corresponding sides are pro¬ 
portional. If either of these statements is true, the other is also true. 
Similar triangles have the same shape, but are not necessarily the 
same size. 

LA--LA' 

LC=LG' 

a_ a' 

h~V 

etc. 

2. Theorem of Pytho.goras, 


b 


The square of the hypotenuse of any 
right triangle is equal to the sum of the 
squares of the. other two sides: 

Figure 2. —Theorem of Pythagoras. 

3. Proportion .—A proportion may be expressed in several forms, 
and restated in several ways. 

a:h::G‘.d is the same as ^ = ^ 



(1) 






2 


It may be restated: etc., so long as the relationship 

ad='bc is preserved. 

A proportion is properly expressed if corresponding parts (not 
necessarily all parts) are in the same unit. 

^ 20" _ x" . 6 mm. 13mm. 

ihus: 79 /’ 7 ' x' 

4. {a) 30-60-90 Triangles.—h\ a 30-60-90 the sides bear the rela¬ 
tions shown in figure 3. 




/ 

Figure 4. 


{b) 45-45-90 Triangles.—hi a 45-45-90 the sides bear the relations 
shown in figure 4. 



OF, length 100, is at 90° to eye and seems to have length 100- 100% 

OF, length 100, is at 45° to eye and seems to have length 70_ 70% 

OF, length 100, is at 30° to eye and seems to have length 50- 50% 


Apparent size varies little for change of angles near 90°. 

Apparent size varies much for change of angles near 0 °. 

With the exception of ordinary arithmetical procedures, no other 
mathematics is necessary. A few equivalents may be helpful: 

6,080 feet =1 nautical mile. 

5,280 feet =1 statute mile. 

1 knot =1.69 feet per second (approximately). 

6,400 mils =360° 

V3_ =1.7321. 

V2 =1.414. 

1° =17.77 mils. 

1 centimeter =0.3937 inch. 

1 inch =2.540 centimeters. 

1 gram =15.42 grains. 

1 ounce =437.5 grains. 















Section 11. INTRODUCTION 

Shooting may be classified as follows: (a) stationary platform and 
stationary target; (b) stationary platform and moving target; 
(c) moving platform and stationary target; (d) moving platform and 
moving target. The aerial free machine gunner belongs in the fourth 
category. The purpose of this manual, therefore, will be to develop 
methods by which his problem may be solved, first, from a theoretical 
standpoint, and second, from a practical standpoint. The instructor 
should realize that aerial sighting is in itself a very complex problem 
of speeds, firing angles, approach angles, etc., but that the apparent 
speed method of sighting is a simple, practical solution of this 
problem. 

The following are some of the air gunner’s problems as compared 
with those of a ground gunner: 

1 . The air gunner’s firing platform is always moving. His 
speed may be as high as 500 knots. The ground gunner usually 
fires from a stationary platform. 

2 . The air gunner’s target is usually a high-speed aircraft 
which is never stationary. Its movement is rapid and irregular. 
Changes are quickly made in direction and speed by the maneuver¬ 
ing of the target aircraft. ‘ The ground gunner’s target is usually 
fixed or slow moving. 

3. The air gunner and his target may move in different planes, 
while the ground gunner and his target are usually on the same 
horizontal plane. 

4. The air gunner has a very difficult problem of range, while 
the ground gunner can make use of range finders, maps, or ground 
reference points to determine range more easily. 

5. The air gunner is mainly on his own as to his fire control 
and conservation of ammunition. His only contact with the pilot 
or fire control officer is by way of the interphone system. The 
ground gunner’s fire is directly controlled by his unit commander. 

6 . The air gunner must act in a split second. He does not have 
the time that the ground gunner has to compute his aim carefully. 

From the above it can be seen that the air gunner must have a much 
greater knowledge and proficiency than the ground gunner. 

It is obvious that in addition to the bead and peep, some ring or 
rings will be helpful in dealing with the gunner’s problem. The 35- 
mil (50-knot) sight is believed to be the best for this purpose. Its 
construction and the advantages to be gained from its use will be 
discussed in the next section. 


(3) 


Section III. CONSTRUCTION OF THE 35-MIL SIGHT 


All Navy free machine gun sights are constructed on the same basic 
principles. In order to explain their construction, certain terms must 
be defined or illustrated. 

Mils^ like degrees, measure the size of an angle. A mil is the angle 
formed at the eye by viewing the two ends of an object 1 foot long at 
1,000 feet. More precisely, an angle of 1 mil is the angle determined 
by the arc of length 1 foot in a circle whose radius is 1,000 feet. The 
curvature of the arc is so slight that from a practical standpoint the 
arc may be considered a straight line. A mil is 1/6400 of the circum¬ 
ference; i. e., 6,400 mils=360°. This arc or line is 1/1000 of the radius. 



Sight axis .—The line from the gunner’s eye through the peep and 
bead projected out into space. 

Line of sight .—The line from the gunner’s eye to the target with 
reference to the ring. This may be outside, through, or inside the ring 
depending on the relative position of the gun and target. 

Angle of deflection .—The angle between the sight axis and the line 
of sight. 



In the simplest case of sighting, the bead of the post sight, seen through 
the peep, is pointed at a fixed target and the barrel of the gun is aimed 
at the target. 


(4) 







5 


In order to aim at a moving target there must be a correction in 
this simple sight to take care of target motion; that is, the target must 
be “led.” This correction may be determined by the use of a ring. In 



all cases, the bead must be centered in the middle of the peep in order 
for the gunner to get the correct angle of deflection. This angle is 
measured with the ring. 

The gunner must keep his eye at a predetermined distance from 
the ring if this angle is to remain constant. This distance is known 
as the “sight base.” 

With this hypothesis, the radius of the ring of the 35-mil sight may 
be determined as follows: 


S 



Figure 9.—Computing the radius of the ring. 


By definition, the angle at A is 35 mils, meaning that d.(7—1,000 
and BC=Zh. If we take 20 inches as our sight base and remember 




















6 


that triangles ADE and ABC are similar, we may compute the radius 
DE of the ring by means of the proportion: 


20 _ 1 , 000 
DE~ 35 


and DE—^A 


Hence, if the sight base is 20 inches,^ the radius of the ring will be 
0.7 inch, or the diameter 1.4 inches. 

By using a similar line of reasoning, this ring may be used in 
estimating range, assuming that the size of the enemy aircraft is 
known. 

As a simple example, assume the wing span of a fighter plane to be 
35 feet. If this plane just fills the radius, its range will be 1,000 
feet; if it fills the diameter its range will be 500 feet. In general, the 
distance from the gunner’s eye to the ring (sight base) is to the range 
as the radius of the ring is to the size of the target. Further discus¬ 
sion of this use of the sight will be found in the section on range 
estimation. 

In addition to using the 35-mil sight for range estimation, it is 
possible to use it in handling the problem of speed. The amount of 
lead necessary in any given case clearly depends (among other things), 
on the velocity of the projectile. This velocity is decelerating, com¬ 
paratively rapidly at first. By studying the velocities at different 
ranges of the .30 caliber M2 ammunition in a BAM gun, we find 
that a good average is 2,414 feet per second. We can introduce this 
into the previous figure: 


B 



Figure 10. —Computing the speed of the ring. 


If we assume that the gunner is at A shooting along AC and 
an aircraft at B is flying toward 6'', it may be seen that at the end of 1 
second the projectile will be at 6*^, assuming that the projectile maintains 


sigl^ing^in ufis^manu^/^^^^^ assumed in the demonstrations and diagrams dealing with 








7 


its average velocity. The aircraft will also be at G at the end of 1 
second provided its speed is 84.5 feet per second. Hence C will be the 
point of aim for the gunner in order to hit the aircraft under the con¬ 
ditions stated. Using the ring will therefore lead the target by a 
proper amount; that is, if the target is put on the ring it will result in 
a collision course between the projectile and the aircraft. Since 84.5 
feet per second is equivalent to 50 knots, this may be called a 50-knot 
ring. The section on apparent speed sighting will discuss this in more 
detail. 

In connection with the use of 2,414 feet per second for the velocity 
of the projectile, it should be noted that the initial muzzle velocity is 
2,700 feet per second and that the distance traveled by the projectile in 
the first second is 2,170 feet. Because of the fact that most effective 
firing will be done at ranges of 1,500 feet or closer, neither of the latter 
two figures would be practical. 2,700 feet per second is only main¬ 
tained for an instant, and 2,170 feet is too great a range. 2,414 feet 
per second is the average velocity up to about 600 feet, and it has the 
great advantage that the 35-mil angle is maintained for range estima¬ 
tion, and the 50-knot ring is maintained for speed estimation. Its use, 
however, is arbitrary. 

Using 2,170 feet and 2,700 feet instead of 2,414 feet would result in 
39- and 32-mil angles, respectively, instead of a 35-mil angle as the 
following figure indicates. 



Figure 11.—Comparison of rings with different velocities. 














Section IV. RANGE ESTIMATION 

Range estimation is the first step that the gunner takes in preparing 
to shoot at a target. It is necessary for the gunner to know the range 

\ 



of the target in order to use either the apparent motion or the tracer 
method of sighting. Above all, it is essential that the gunner know 
whether the target is within the effective range of the gun or not so that 

(8) 

















9 

he will not waste his limited supply of ammunition when the chance of 
success is negligible. 

The size of the target as it appears in the ring gives an accurate 
estimation of the range, providing the gunner knows the wing span or 
fuselage length. The importance of aircraft recognition cannot be 
overemphasized in connection with his work. 

In using the ring sight to estimate range, the gunner must know 
the size of the target. This knowledge will result from constant asso¬ 
ciation with the various types of enemy aircraft and complete famil¬ 
iarization with the wing span and fuselage length of these planes. 
Knowing the size of the target, the size of the ring, and the distance 
from the eye to the ring, the gunner can determine range by noting the 
portion of the ring filled by the target. Figure 12 demonstrates this 
point. 

In this figure, the target is an enemy fighter whose wing span is 35 
feet. In position A, the range is 500 feet as the wing span fills the 
diameter of the ring. 

In position the same fighter is seen to be at 1,000 feet because it fills 
one-half the diameter (one radius). It appears to be half as large as 
in position A, 

Going back to the ‘‘Construction of Sights” we find that the 50-knot 
sight is composed of a ring 1.4 inches in diameter and that the sight 
base for this sight is 20 inches. With these two known factors plus 
the wing span or fuselage length of the target it is possible to deter¬ 
mine the range by setting up a simple problem of proportional 
triangles. 





Thus, by comparing these similar triangles ABC and ADE^ the 
distance from the gunner'’s eye to the ring (the sight base) is to the 
range as the size of the ring is to the target. 

By substituting the above figures we obtain— 

35 

X 1.4 
1.4,??=700 
a?=500 feet 











10 


Therefore, if any enemy aircraft with a wing span of 35 feet com¬ 
pletely fills the 1.4-inch ring, the gunner knows that the range of this 
plane is 500 feet. 

In this same manner, if the enemy aircraft fills of the ring (the 
radius), the range is 1,000 feet. If it fills a space % the size of the 
ring the range is 1,500 feet. 

As an additional demonstration let us consider another problem 
of the same type but with the unknown “a?” representing the space 
the target fills in the ring sight rather than the range. 

What proportion of a ring sight whose diameter is 1.4 inches would 
an airplane with a wing span of 40 feet fill at a range of 1,500 feet? 



Figure 14.—^Determining the amount of ring filled by a plane of known size at known 

range. 

Using the same formula we used in the last problem we obtain the 
following : 

20 X 

1,500^40 

l,500aj=800 

a? =.533 inch 

This amount is approximately % the radius of the ring, so we can 
conclude that a plane with a wing span of 40 feet at a range of 1,500 
feet fills approximately % the radius of the ring. 

The ranges that a gunner must recognize are 2,000 feet, commence 
apparent motion calculation; 1,500 feet, maximum^ effective range; 
1,000 feet, effective rang eg 500 feet, foint-blanh range. 

By solving the above problem for ranges of 500, 1,000, 1,500, and 
2,000 feet for both the radius and the diameter of the ring, we will 
obtain the following figures: 


Range (feet; 

Size of tar¬ 
get that will 
fill radius 

Size of tar¬ 
get that will 
fill the 
diameter 

500__ 

17.5 

35 

1,000_ 

35 

70 

1,500_ . 

52.5 

105 

2,000_ 

70 

140 
















11 


It may be noted that inverse proportion is involved. If the target 
looks twice as big, it is half as far; if it is % as big, it is 4/3 as far, 
etc. 

From the wing spans of the enemy fighters, we can roughly 
conclude that a single-engine fighter (approximately 35 feet) will fill 
two radii (the diameter) of the 50-knot ring at 500 feet, the radius 
at 1,000 feet, % of the radius (I /3 of the diameter) at 1,500 feet, 
and the radius at 2,000 feet. Also, a two-engine fighter (52.5" wing 
span) will fill 3 radii at 500 feet, I-II /2 i‘adii at 1,000 feet, the radius at 
1,500 feet, and % of the radius at 2,000 feet. Finally we can roughly 
conclude that the wing span of a twin-engine light bombing plane 
(approximately 70 feet) will fill four radii (twice the diameter) at 
500 feet, 2 radii (the diameter) at 1,000 feet, % of the radius (% the 
diameter) at 1,500 feet, and one radius at 2,000 feet. 

In order to judge range more accurately, the gunner may use the 
following method: recognize the enemy aircraft, and its size—e. g.. 
Me 109—span 32 feet; then mentally note the size this plane will 
appear in the ring at 1,000 feet (effective range). 

This will be 32/35, or roughly 90 percent of the radius. This size 
should be carefully fixed in the gunner’s mind because it is at this 
range, 1,000 feet or closer, that most effective fire will be delivered. 
The next step the gunner should make is to note that this target 
will be approximately I /2 size or 45 percent of the radius at 
2f}00 feet— 2,000 feat being the range at which the gunner should start 
to estimate apparent motion. It is expected that due to the mental 
delay of the gunner he will not actually estimate the apparent motion 
until around 1,500 feet, the maximum range for effective fire. 

Students should practice range estimation throughout their careers 
as gunners. This can best be done with a gun or a 20 -inch sight bar 
with the 50-knot ring. For group instruction the use of %o scale 
models is invaluable. A %o scale model at %o distance will appear 
the same in the sight as a full-size model at the full-scale distance. 

The range-estimation range is illustrated in figure 15, on page 12 . 

The instructor can have the scale model carried back and forth 
in this area and can very accurately check the student’s range estima¬ 
tion by the reference lines drawn on the deck. Particular emphasis 
should be placed on ranges of 1,000 and 2,000 feet. 

In order for range estimation to be of any value under combat 
conditions, it is necessary for all gunners to practice until they can 
recognize ranges without consciously going through all the steps 
listed in this paper. 

In case scale models are not available, any scale can be used. 
The most common scales at present are 1/24, 1/36, and 1/72. What- 


493667 °—43 -2 


12 



ever scale is used in the model must also be used in laying out the 
range markings on the deck. In using scale models the student should 
remember that the range is measured from the eye to the target. 
A small error in measuring the scale range will give a large error in 


Figure 15. 

the range-estimation result. For this reason 1/72 scale models are 
not recommended. With these models a 10-inch error in the scaled 
range will give the student the same effect as a 72- by 10-inch (60 feet) 
actual range error. 

Air gunners should familiarize themselves with the size of any air¬ 
craft with which they will come in contact. A gunner doing duty 
in Hawaii would be interested in only the Japanese planes, while a 
gunner operating in northern Africa would be interested in German 
and Italian planes in use in that area. 













Section V. THEORY OF FREE GUN SIGHTING 

In considering the problem of the free gunner, which is to shoot 
from a moving platform at a moving target, it will be helpful to 
consider first the separate problems: Shooting from a fixed platform 
at a moving target, and shooting from a moving platform at a fixed 
target. In either case, the gunner does not aim directly at the target, 
but picks a spot somewhere ahead of or behind the target. We shall 
disregard for the present any slowing down of the bullet during 
its flight. 

1. FIXED PLATFORM, MOVING TARGET 
When a gunner shoots at a moving target, he must realize that his 
bullet does not reach the target instantaneously, but that it takes a 



certain definite time which depends on the range and the bullet’s 
velocity. During this “time of flight” of the bullet, the target moves 

(13) 







14 


through a certain distance* which depends on its speed. The gunner 
must estimate this distance and shoot ahead of the target to allow 
for it. This correction is called target de-flection or lead. 

Target deflection or lead is not a mysterious correction used only 
in aerial gunnery. It is an everyday occurrence which nearly every- 



Figuee 17.—Hunter shooting at a duck. 


one has observed or used almost unconsciously. A common example 
is a stationary passer throwing a football to a runner crossing in 
front of him on the football field. He does not throw it at the runner, 
but always at some point in front of him in such a manner that the 
runner can catch the ball without changing his speed or direction. 

Both a football and a bullet require a definite time to cover a given 
distance. This cannot be disregarded in gunnery or the result will 
be consistent missing of the target. 











15 


It will be well to consider in more detail the problem of lead and 
take the case of a hunter shooting a duck on the wing. In discussing 
target deflection we will disregard the fall of the shot caused by gravity 
because we shall see later that it is of minor importance in aerial 
gunnery. Figure 17 shows a hunter shooting correctly at a duck flying 
across directly in front of him. 

Assume the distance AB from the hunter to the duck is 100 feet 
and that the velocity of the shot is 1,000 feet per second. It will take 
one-tenth of a second for the shot to reach the duck. If the duck 
is flying 50 feet per second, it will fly 5 feet during the time of flight 
of the shot. Hence the duck will fly from B G while the shot 
goes from A to C, Now if the duck were twice as far away, that 
is at Z>, and flying at the same speed, the shot would take one-fifth 
of a second in flight (from A to E) and the bird would fly from 
I) to requiring a 10 foot lead. It is important to note that in each 
case the angular lead (angle of deflection) is the same and is inde¬ 
pendent of range. In this case, it should be observed that the target 
is traveling at approximately a right angle to the gunner’s line of 
sight. This is called a full deflection shot. The angle between the 
line of sight of the gunner and the target’s line of flight is called 
the approach angle.. An approach angle of 90° is threfore a full 
deflection shot. It should be obvious that if the duck were coming 
directly toward or going directly away from the hunter (flying 
along AZ>), no angular lead would be necessary. It would then be a 
“no deflection” shot, and the approach angle would be 0°. 

If the target is approaching (or departing) at a 45° angle to the 
line of sight (45° approach angle), we may determine the correct 
lead by the aid of the following figure: 



As before, while the shot goes from A to G {A to O') the duck 
goes 5 feet from B to G {B to G') but the lead apparent to the 
gunner is the line GD {G'D'). This may be determined by solving 
the triangle BDG. This is a 45-45 right triangle whose hypotenuse 
is known. 

2X-=-25 
X =3.5-}- 





16 


Instead of leading the duck 5 feet as we did in the 90° approach 
angle, we lead it about 70 percent of 5 feet or 3.5 feet in this case. 
(In the 45-45 right triangle a leg is always about 70 percent of the 
hypotenuse.) If the duck were departing at 45°, the correct lead 

C 


P 

Figure 19.—The 45-45 right triangle. 

would be D'C'. The difference in the ranges AB^ AC and AG' 
can for all practical purposes be overlooked because of their small 
size as compared with the muzzle velocity. 

From the foregoing we can see that we must allow for a full de¬ 
flection shot (100 percent lead) when the approach angle is 90°, 
70 percent lead when the approach angle is 45° and no lead when 
approach angle is 0°. A 30° approach angle will similarly be found 
to require a 50 percent lead. It should be noted that the greatest 
change in lead takes place as the approach angle changes from 
0° to 45°. 

In summary: 


Approach angle 

0° 

30° 

45° 

60° 

90° 


Lead 

0% 

50% 

70% 

85% 

100% 




It is very important to realize that for a given target speed, range 
has no effect on deflection. Deflection as applied on an aircraft ma- 
chinegun sight is angular. 



2. MOVING PLATFORM, FIXED TARGET 

We shall now consider the effect of the motion of the aircraft on the 
path of the projectile when a gun is fired from it at a stationary target. 
In the first instance, let us assume that the axis of the bore is at right 
angles to the flight axis of the aircraft. The angle between the axis 
of the gunner’s bore and the line of flight of the gunner’s aircraft is 





















17 


known as the firing angle^ so in this case, the firing angle is 90°. Be¬ 
cause of the forward movement of the projectile as it leaves the gun, 
the projectile will follow a path which will be the resultant of the for¬ 



ward speed of the aircraft and the muzzle velocity of the projectile. 
This may be shown by the use of vectors. Suppose the aircraft is going 
100 knots (169 feet per second) and the velocity of the projectile is 



2,414 feet per second. Figure 21 shows the trajectory of the projectile. 

If the gun is fired when the bore is pointing directly at the target, 
the bullet will strike at a distance equal to one second’s travel of the 





















18 


aircraft (moving platform) ahead of the target on a line parallel to the 
firing plane’s line of flight. 

Hence, when the firing angle is 90°, the gunner must point his gun aft 
along the line of flight the full deflection (100 knots in this case). This 
correction is called gunner^s deflection or lag. 

Figure 22 shows a marine throwing a grenade from a jeep at a 
machine gun nest. Notice that the point of aim is a considerable dis¬ 
tance behind the target, and that the grenade is carried forward by the 
motion of the jeep. 

When the firing angle changes, the gunner’s deflection or lag will 
change in the same way that the target deflection or lead changed as 
the approach angle changed. When the firing angle is 0° (firing 
straight ahead or dead astern), no gunner’s deflection is necessary. 
Figure 23 will illustrate a firing angle of 45°. 

If the gun is pointed at 5, the bullet will be carried forward by an 
amount equal to the forward speed of the aircraft and it will land at G. 



Figure 23,—Firing angle of 45°, 


The real lag is BG^ but what the gunner sees is the apparent lag BD^ 
so this is the correction that should be made. If the speed of the air¬ 
craft is known, this apparent lag may be computed by the solution of 
the triangle BDG. This is a 45-45 right triangle, so as before, BD 
about 70 percent of BG. It may be shown similarly that when the firing 
angle is 30°, it is a 50 percent lag, etc. Precisely the same table will 
apply for firing angles as applied for approach angles. (See p. 16.) 

The gunner should note that these firing angles are for any direction 
in relation to the plane’s axis of flight. 

3. MOVING PLATFORM AND MOVING TARGET 

We are now in a position to consider in theory the actual problem 
of the free gunner. It will, of course, be a combination of the two 
situations just discussed. It should be remembered that lead or target 
deflection should be computed first and from this point, lag or gun¬ 
ner’s deflection should be determined. Lead will always be along the 
line of motion of the target and lag will be along a line parallel to 
the line of motion of the gunner’s plane. 


19 


As a simple example, suppose two aircraft are flying a parallel 
course at the same speed. The approach angle being 90°, the gunner 
should give a full deflection lead. Because the firing angle is also 
90°, there is also a full deflection lag. Hence in this instance, the 
gunner will aim directly at the target. 

If, as a second example, the gunner’s plane was going 150 knots 
and the target plane 100 knots and they were in the position shown 



Figure 24. —Two aircraft on parallel courses at same altitude. 


in the figure 25, it may be seen that the approach angle is 90° and 
a full lead, 100 knots, should be given the target. The firing angle, 
however, is about 45°. Hence it is 70 percent of 150 knots deflection 
or 105-knot lag as seen in the sight. The point of aim is shown in the 
figure. 

It should be perfectly obvious that a gunner in the air will be unable 
to do all the reasoning and calculating indicated in this section. We 



have seen that the speed of the target, speed of the gunner’s aircraft, 
the approach angle and firing angle all have to be considered if the 
proper correction is to be made in the sight. The gunner should un¬ 
derstand, however, why these corrections exist chiefly for the purpose 
of convincing himself that sighting corrections are necessary, and 
that the instances are few where the gunner should fire when aiming 
directly at the enemy aircraft. 

The student must realize that the actual speeds of the two aircraft 
cannot be combined to solve the sighting problem, but rather that the 










20 


vector components can be used to determine an apparent speed. By a 
close study of the above sighting problems, it can readily be seen that 
the resultant lead and lag can easily be combined into a single line of 
apparent motion in the sight. 

Two methods of sighting have been developed which have turned 
out to be very practical and which are far simj^ler for the gunner to 
use. They are the apparent speed method and the tracer method^ the 
first of which will be taken up in the following section. 


Section VI. APPARENT SPEED SIGHTING 


Apparent speed sighting is a method by which the gunner can judge 
the proper lead for a moving target when firing from a moving air¬ 
craft. 

When using this method, the air gunner is concerned with the ap¬ 
parent movement of the enemy across the sight ring. He determines 
the apparent speed and apparent direction of movement of the enemy 
aircraft with the aid of his sight rings, and applies the proper deflec¬ 
tion. This deflection is not always along the actual flight path of the 
enemy, but is always in such a direction that if the gun were held 


A 



stationary while applying the correct deflection, the enemy would 
appear to move toward the center of the sight. 

In the “Theory of Sighting” section, the simple problem of firing 
at a target flying at the same speed and in the same direction as the 
firing plane, w^as considered. 

In order to hit the target, the gunner aims directly at it, that is, 
lead and lag offset each other. The projectile and the target will 
meet at the point “A.” In this case (fig. 26), there is no apparent mo¬ 
tion of the target in the gunner’s sight if he holds his gun steady in 
relation to his own airplane. 

Thus, a gunner can conclude that if he holds his gun steady in rela¬ 
tion to his own airplane and he sees no apparent motion of the target 
i]i his ring sight, no lead is necessary. 

Figure 26 illustrates the very unlikely sighting problem of a target 
flying “in formation” with the firing plane. Normally, a large per¬ 
centage of the sighting problems of a free gunner against a high-speed 
fighter have much less than full deflection. However, the gunner must 

( 21 ) 






22 


know thoroughly the apparent speed method of firing, and when he 
does have a no-deflection shot his problem will only be easier. Actually 
there will seldom be a no-deflection shot. 

The attacking plane will make your problem difficult by following 
a curved track rather than a simple straight tail chase, and of course 
your own plane will be maneuvering. It has been found that most 
effective attacks are delivered from a comparatively small sector 
around the tail. That does not mean that attacks will all be delivered 
from that sector, or that you wdll not have many good shots at an 
enemy attacking an airplane in your formation. Remember that an 
enemy attacking from astern will always have apparent motion if he 



is firing at you, unless he is flying dead astern on your course at the 
same altitude. 

At this point it is well to consider the two terms relative speed and 
apparent speed. If two aircraft are flying at the same altitude on 
parallel courses at 150 knots and 100 knots, respectively, their relative 
and apparent speeds are 250 knots if flying in opposite directions, and 
50 knots if flying in the same direction when abeam. If the planes 
are in the position shown in figure 27, the relative and apparent speeds 
will be different. 

In figure 27, the target plane {T) is flying 250 knots along AG^ and 
the gunner’s plane {G) 225 knots along GL. Using vectors, AG rep¬ 
resents speed and direction of plane T. If we consider a wind of 225 
knots blowing in the direction R8^ plane G will theoretically be sta¬ 
tionary. But this wind will act on plane T along AD, The resultant 











23 


of these two forces will of course be found by completing the paral¬ 
lelogram. This resultant force is AB and is the relative speed. This 
vector AB represents both the speed and direction of movement of 
plane T in relation to plane G. In this case the relative speed is 125 
knots, although the method of obtaining this is not important. 

As seen by the gunner in plane G^ the line AB doesn’t represent 
either the speed or direction because to the gunner in plane G^ plane T 
appears to be going in the direction AM. 

This movement along AM at right angles to the line of sight is the 
apparent speed.^ which when applied in the sight actually corrects 
for the relative speed. In the above example it is 60 knots, considerably 
less than the relative speed. It should be noted that the apparent 



Figure 28. —Line of apparent motion. 


speed for which the gunner allows always is at right angles to his 
sight axis. 

If there is apparent motion evident when you have the enemy in 
the center of your sight, hold your gun steady and observe this move¬ 
ment out away from the center. This movement establishes an imag¬ 
inary line, and it is vital to your problem of getting hits. 

In figure 28, the line of apparent motion is AB. Consequently the 
gun must be pointed at G. The gunner must always point his gun 
along the line of apparent motion, ahead of the target.^ if he expects 
to make hits. There must be no mistake about this point. 

The amount which he leads the target, as determined by the ap¬ 
parent speed, is estimated by timing the movement of the target across 
the ring for the existing range. 

It must be realized that the motion across the ring (actually it is 
angular velocity), cannot be used to compute speed unless range is 
considered. For example, an airplane a mile away appears to move 
slower than when it is half a mile away. At 1,000 feet it literally 


24 


flashes by. At 200 feet, its apparent speed is tremendous, while of 
course its actual speed has remained the same. Range estimation is 
covered in other sections. Establishing the apparent motion line has 
been covered above. Apparent speed is estimated as follows: 

The radius of any ring of any type ring sight subtends definite 
distances depending upon the range. The radius of a 35-mil ring 
subtends 35 feet at 1,000 feet, TO feet at 2,000 feet, etc. 

If a target whose range is 2,000 feet moves across the radius 
of the 35-mil ring in 1 second, we know it has covered 70 feet. A 
knot is 1.69 feet per second. Therefore the apparent speed is 41 
knots. If the range is 1,000 feet and the target moves across in 2 
seconds, the apparent speed is 10.5 knots. 

It is of interest to consider the maximum apparent speed with 
which a gunner can contend. For one gunner and one type 
installation it may be more than with the same gunner in another 
installation. It may vary with different gunners in the same 
installation. And of course it should be realized that a gunner 
who can just follow a target with 100 knots apparent speed at 
1,000 feet will not be able to do so at 500 feet. 

Speed Table.— Speed in knots at which target must move to cross the radius of 
the 35 mil Hug sight at various ranges and for various times 


Range 

500 feet 

1 1,000 feet 

1,500 feet 

2,000 feet 



Sketches show a 35' plane at various ranges 

Distance covered by- 
ring radius 



<3 

Q 



17.5 feet 

35.0 feet 

52.5 feet 

70.0 feet 

Time in , 
seconds 

' Seconds 
.5 

1.0 

1.5 

2.0 

2.6 

3.0 

3.5 

Knots 

21 

10 

7 

6 

4 

3 

Knots 

41 

21 

14 

10 

8 

7 

6 

Knots 

62 

31 

21 

16 

12 

10 

g 

Knots 

83 

41 

28 

21 

17 

14 

12 

11 


4.0 


5 

g 







The table above gives distances subtended by the 35 mil sight 
radius at various ranges, and the speed at which a target must be 
traveling in order to cross the radius in various half-second intervals. 
Gunners should be trained in estimating these time intervals by use of 
a metronome or some other satisfactory device. An interval of !/> 
second is about the smallest that gunners can be expected to estimate 
accurately. 

















25 


The apparent speed is allowed for by use of proportional parts of 
the 35-mil sight, assuming that the full ring radius allows for a 50- 
knot apparent speed. 

The free gunner should realize that in attacks by an enemy on his 
aircraft, the apparent speed will vary. In the majority of the situa¬ 
tions the apparent speed will diminish in proportion to the range as 
the range decreases, because the fighter aircraft must fiatten out his 
attack in order to hold the attacked plane in Ms sight. It is not 
possible to maintain a collision course of the target and bullet without 
changing the attitude of the gunner’s plane because the bullet speed 
is so much faster than the speed of the attacking plane. 

In figure 30, a common attack is illustrated. Because both planes 
are moving, the target will appear to “suck in” until the attack ends 
up with a no defiection shot at close range for both the target and the 
firing plane. The gunner should notice that the apparent motion 
represented by the lines AB becomes less and less as the attack 
progresses. 

The gunner should understand that apparent motion is “not ap¬ 
parent” when the gunner is properly leading and following through. 
For this reason, all gunners should be cautioned to check the line of 
apparent motion and the apparent speed at least once in each attack. 
In an attack as illustrated in figure 30, the gunner must slowly reduce 
his lead as the attack progresses. 

It should be noted that in attacks as illustrated in figure 30, the 
target’s approach angle is almost 0°. In a very high percentage of the 
attacks (approximately 85 percent) now being made by enemy air¬ 
craft, the approach angle, inside effective ranges, is less than 20°. 

The following is the itemized list of the steps taken in using the 
apparent speed method: 

{a) Recognize the enemy aircraft, noting the size it will appear at 1,000 feet 
and 2,000 feet. 

(&) At 2,000 feet place the target in the center of the bead and peep. Hold the 
gun steady in relation to your aircraft. 

(c) Note the direction in which the target moves. It is along this line that the 

lead must be made. 

(d) Time the target across the ring’s radius (at the same time as (C7)). 

(e) When the target reaches approximately t,500 feet, open fire using the 

calculated lead. Reduce lead slowly. 

(f) At 1,000 feet, or when target makes radical maneuver, stop gun for an 

instant in order to observe direction of apparent motion and to check 
apparent speed. Do not allow target to go all the way across the ring. 
Reopen fire. (Lead is normally about one-half of original if attack is 
similar to figure 30.) 

(^r) At slightly less than 500 feet shift to point blank fire. Aim at nose of 
enemy aircraft. 


26 



FiGi’itK 20.^—StaiidaJcl iiigh side attack on bomber. 








































































































































































































































































































































































































































































































































































































27 


{h) When the plane breaks away shift point of aim to the proper lead along 
the line of apparent motion. 

It is believed that planes attacking in a pursuit curve will conform 
to the following characteristics in their breakaway : 

(a) There will be a slight hesitation of approximately 1 second between the 
time the pilot makes up his mind to break away and the time that the 
plane actually starts into its new attitude. 

(&) Planes breaking away in a violent skid may be expected to have a line of 
apparent motion which is the line of the wings extended in the direction 
of the breakaway. 

(c) Planes breaking away by pulling up, pushing over or turning to either side 
may be expected to have a line of apparent motion which is an extension 
of the line of the fuselage. The amount of lead necessary will vary in 
each case and in all likelihood with each pilot and each type of plane. 



id) For firing at the commencement and during the breakaway, it is recommended 
that (]) point blank fire be continued until the plane has actually entered 
the breakaway, and (2) that the gunner then shift his point of aim to 
the proper lead along the line of apparent motion increasing or decreasing 
as necessary to keep his bullet pattern on the target. The most important 
point, of course, is to keep the bullet pattern on the line of apparent motion 
and next to maintain the proper lead. Assuming that the original lead 
used is correct, the number of hits will depend upon how closely the gunner 
maintains the proper lead. It is obvious that if he trains only half as fast 
as he should have, he will get twice as many hits as he would have if he 
held the gun still and let the plane fly through the bullet pattern. There¬ 
fore, the number of hits which he obtains will depend upon how closely 
the rate of training his guns follows the speed of breaking away. 

The Two-Thirds-of-a-Second Method (“Elephant”)- 

This same theory is often used in a slightly different method. In 
this method a standard fixed time is used (two-thirds of a second). 
In order to estimate this time the gunner says the word “Japanese,” 
“elephant,” or some similar word that he can say in approximately two- 
thirds of a second. The projectile travels approximately 1,800 feet 

493607°—43-3 







28 


in this time. So, if the gunner observes the distance the target travels 
across his ring in this time, he can use this correction on his ring to 
lead the target when it is at a range of 1,800 feet. 

The following are the steps used in this method: 

(1) Recognize aircraft and move gun so as to hold enemy in center of sight 
until range is 2,000 feet. 

(2) Stop motion of gun and observe how far the plane moves across the sight 
ring in two-thirds of a second. Also observe the direction of motion. 

(3) Put plane same distance on opposite side of sight and fire at will (decreas- 






Figurk 31.—The “Elephant” method. 

ing lead gradually as plane moves toward the tail of your own plane), until enemy 
is at 500 feet; then fire point blank. 

(4) When the plane breaks away shift point of aim to the proper lead along 
the line of apparent motion. 

In estimating apparent speed, both the two-thirds of a second 
method and the timing from peep to ring method have their advantages 
and their weaknesses; each method has its adherents. The objective, 
however, of the two methods is identical: namely, to develop an in- 
stinctive sense for the proper lead, an instinct which should be so 
deeply engrained in the free gunner before he reaches the fleet that 
it will not desert him under the stress of combat. Whether either 
of the timing methods could be used in actual combat is a matter on 
which there has been some discussion, but the practicality of such 


29 


procedure is doubted. These methods should be regarded primarily 
as training techniques, a framework upon which the instinct for lead 
is built. Few recruits who have never shot at a moving target, or 
have shot very little, have this instinct, and it can only be developed 
through constant practice. During this period of development, how¬ 
ever, a definite technique for estimating apparent speed is valuable. 

If two planes are following curving courses, a rather different situa¬ 
tion develops. 

In figure 32, it is assumed that the two planes at A and D are flying 
at the same speed and are always diametrically opposed on the circle. 
If a gunner in D looked through his sight, he would see no apparent 
motion of A. Under these circumstances, applying the usual pro¬ 
cedure he would aim directly at If we consider lead and lag sep- 


A 



Figure 32.—Two planes on circular course. 

arately to determine the point of aim, it may be noted that the correct 
lead would be along the circumference AB. The lag would be along 
the straight line BC. The forces set up as a result of the gunner's 
plane remaining constantly on the circumference will not affect the 
projectile and the lag will therefore be along a line parallel to the 
flight axis of the plane, but in the opposite direction. The correct 
point of aim is therefore C. It may easily be seen that in this case, 
the apparent speed method does not hold. This will be true in general 
when the courses of the two planes are curved lines. 

It is well to point out the sighting errors that may arise when using 
the ring and bead sight. They are four in number: 

(a) Placing the eye at an incorrect distance from the ring sight. 

(&) Not aligning the bead in the center of the peep. 

(c) Improper focus of the eye. 

(d) Failure to estimate range and apparent .speed correctly. 

(e) The 35 mil sights are designed so that the ring is the proper size for the 

sight base involved. If the sight base designed for a particular sight 
is not maintained, a large error in range and speed estimation will occur. 




30 


Placing the eye 2 inches too close to the- sight will cause an error of 
3.9 feet at 1,000 feet. 

These errors in range will of course result in errors in apparent 
speed estimation. In order to keep these errors as slight as possible, 
it is necessary for the gunner to check on his sight base. This 



Figure 3H.—Eye too far from ring. 


distance is obtained by resting the chin on the hand holding the left 
spade grip. . 

Improper alignment of the bead and peep will, of course, change the 
relationship of the line of sight to the axis of the bore and this will 
add an error. 

In addition to this obvious error of misalignment there is a per¬ 
sonal error which occurs. A person reading the time by a large clock 



from directly in front of the clock might read the time as 4 o’clock; 
however, one reading the time from the left side of the clock will 
possibly read it as 2 minutes after 4 o’clock. The true reading is 
obtained only when the observer stands directly in front of the clock. 
To align the bead in the peep correctly, the eye must be on the sight 
axis. It is possible to keep the bead in the peep and move the eye 
about 1/4 of an inch. This %-inch error will cause an error of about 
12.5 feet at 1,000 feet. 

Proper alignment can be held by the gunner resting his chin on his 
hand holding the left spade grip. 










31 


The focus of the eye when using the ring and bead is difficult for 
most gunners. The ring is 20 inches distant, while the target might 
be 2,000 feet. This difference in range causes either the ring or the 
target to blur, depending on which one the gunner has focused his 



eye. It is for this reason that the 20-inch sight base is used. This 
difficulty is much more pronounced with shorter sight bases. 

These above errors should be carefully understood by the gunner 
because it is a natural tendency for most gunners to pull their eye 


. . 

\m - /OOO' - 

Figure 36.—Improper focus of eye. 


V 
1 '? 

15 ! 

-J i. 


away from the line of sight once firing has commenced. This is 
caused by flinching or by lifting the head to watch the effects of the 
fire. No matter what method of sighting is used the gumner must not 
lift his eye from the line of sight. 







Section VII. TRACER 


As a result of experience gained in the 1914-18 war, tracer as an 
aid to aerial sighting was abandoned. Two difficulties were apparent. 
First, use of tracer misled the gunner into thinking that he was hitting 
the target when in fact he was not. Second, flight speeds were then 
low enough to permit the use of an alternative method of sighting for 
free guns. The recent reintroduction of tracer is primarily the result 
of increased flight speeds which have made methods dependent upon 
automatic allowance for the gunner’s own air speed less practical. 

The new use of tracer has been developed by the Royal Air Force 
during the present war. A professor of mathematics at Oxford 
University first conceived of the idea in order to obtain greater accu¬ 
racy in aerial firing. The old system of the gunner making an esti¬ 
mation of lead from his own knowledge and experience in actual 
shooting at moving targets was not effective enough. Inexperienced 
gunners had difficulty learning to lead targets properly, as there was 
no standard method at the beginning of the war by which a gunner 
could be taught to estimate proper lead. As has already been shown, 
the apparent speed sighting method is one answer to this problem, and 
this section will discuss the use of tracer as a second solution. Under 
modern combat conditions, tracer can be a real help to the gunner 
who knows how to use it. There are certain false impressions that 
may easily arise. Every gunner must be sure that he understands 
them thoroughly before he attempts to fire with the aid of tracer. 
These will be explained in the course of this discussion. 

The tracer method makes use of luminous bullets. The bullets 
contain a chemical substance which is ignited when the bullet is fired 
from the gun. The light which is given off makes the bullet visible 
to the gunner within the effective range. It should be understood that 
the modern tracer bullet takes the same path as the regular bullet, 
and that a tracer hits just as hard and is almost as effective. Ordi¬ 
narily, only one out of every five bullets is a tracer, but this, of course, 
can be varied at will. 

The gunner will perehaps shoot with two types of trace, the long trace 
and the short trace. The long trace has had rather wide use in this 
country. This trace burns out at 5,400 feet and is of only limited value 
as will be pointed out presently. The short trace burns out at 1,800 
feet and gives the maximum aid to the gunner. 

( 32 ) 


33 


With either type of trace, the first step is to fire a short sighter burst. 
This simply means placing the bead and peep on the target and swing¬ 
ing the gun to follow the target as the burst is fired. 

If, as in figure 37, a gunner at G fires at a target at E which is mov¬ 
ing from left to right relative to the gun, a single bullet of the burst 
will occupy the successive positions 1, 2, 3 when the target occupies 
the positions C. In aerial gunnery, the background to the 

target is generally uniform, and the gunner whose eyes are following 
the target through the ring gets the impression that the bullets swerve 



in the opposite direction to the motion of the gun. This is called 
cut-hack. This is illustrated in figure 38, the bullet appearing to pass 
successively through the positions 1, 2, 3. This same effect may be 
observed by watching the stream of water from a hose as it is swung 
slowly around. It is very important to note that tracer cannot be 
effective unless the gunner looks through the ring at all times. Unless 
he does, the true cut-back will not be seen. When the gunnner looks 
through the sight of a free gun, the sight axis and the axis of the bore 
are close together. Consequently in firing tracer, the bright part of the 
trace will appear to rise almost instantaneously to the sight center. 
The rest of the trace appears not as a continuous streak, but as a row 
of points of light which are moving rapidly away and becoming 
fainter. If the gunner lifts his head only slightly, he will get an 
entirely erroneous impression, since he will see the bright part of the 
trace rise considerably above the sight center. Figure 39 shows the 







34 


trace as seen through the sight with the eye in the correct position, 
while figure 40 shows what happens when the head is slightly raised. 

When viewed from above, the sighter burst appears as a series of 
bullets going along straight lines and, of course, no cut-back is visible. 
Figure 41 illustrates this. 

Tracer cannot be used as though it was a stream of water from a 
hose. This procedure known as hosepiping always results in very bad 



Figure 39.—Tracer as seen through the sight. Figure 40.—Effect of raising head slightly. 

shooting. The firepower of a single or double flexible gun is too small 
for an effective spray. The gun must be aimed correctly for each 
burst. 

From the discussion so far, it may be seen that use of tracer will 
indicate the proper direction in which to lead the target since the 
amount of cut-back and its direction are directly determined by the 
direction and speed of movement of the gun in following the target. 



This in itself is a real aid to the gunner. Long trace, however, cannot 
give the magnitude of the lead. With such trace, the proper placing 
of the target on the trace would have to be done by the apparent 
speed-sighting method. The end of such trace will appear at varying 
distances owing to atmospheric and light conditions; that is, the gun¬ 
ner might be able to see only 2,000 feet rather than 5,400 feet. If 
the gunner then assumed that the trace was 5,400 feet, but he could 
actually see only 2,000 feet, the error would be great if he attempted 
to determine the correct sighting allowance with this trace. It must 
he remembered that long trace indicates only the direction of lead^ 







35 


hut not its magnitude. The limitations in the use of long trace must 
be constantly borne in mind. 

The ideal condition for a trace is that it disappear at approximately 
the maximum,effective range of the gun and also at a distance that is 
always visible to the gunner. With this end in view, the British 
developed the short trace (1,800 feet), which has turned out to be 
much more satisfactory. Before going into the use of this trace, one 
of the weaknesses of tracer firing should be pointed out. 

The fact that the trace appears as a row of points of light diminish¬ 
ing in intensity and that the background in the air is all very similar 
makes it almost impossible for the gunner to estimate range accurately. 
In fact, there is an optical illusion which makes the gunner think that 
his bullets are reaching the target long before they have actually gotten 
there. What actually happens is that the bullets pass between the 



As seen through sight bullets appear to 
hit enemy plane. 



As seen from above bullets pass to rear 
of plane. 


Figure 42. 


gunner and the target, and as the gunner is unable to estimate the dis¬ 
tance the bullets have traveled, he believes that they have passed 
through the target (fig. 42). When this error occurs, the gunner has 
no visible evidence of his mistake and is therefore unable to correct for 
the error. 

In using the 1,800-foot trace, the gunner will observe that the end of 
the trace will be at the same spot with reference to the ring throughout 
the sighter burst. This will be the 1,800-foot point on the trace. 
Other distances may be estimated by taking the proper proportion of 
the distance from the sight center to the end of the trace. This is be¬ 
cause it is assumed that the gun is moved at a constant rate. Because 
of the fact mentioned earlier that the bright part of the trace rises 
almost instantaneously to the sight center, this part of the trace may 
for all practical purposes be neglected. Figure 43 shows the trace 
with the distances marked on it. 

In all cases, distances on the trace are found by considering the sigKt 
center zero, and the end of the trace 1,800 feet. 




36 


It should be pointed out that the greater the distance from the sight 
center to the end of the trace, the more important accurate range esti¬ 
mation becomes. The best situation would be when the allowance to 



Figure 43.—Short trace. 


be made is so small that it is not much greater than the radius of the 
bullet group. If the allowance is small, the whole trace then occupies 
but a small part of the sight ring, and even large errors in range estima- 



Recognize aircraft. 



At 1,500 feet put enemy % of way 
from sight center to end of trace and open 
fii'e. 



At 1,800 feet fire short sighter burst 
and notice end of trace. 




. Figure 44.—A typical run with short trace. 

tion can be covered in the spread of the bullets. Conversely, with a 
large alloivance, accurate range estimation becomes absolutely essential. 
I his principal is of general application in all sighting problems. 







37 


The rules for using 1,800-foot tracer can be given as follows: 

1. Recognize aircraft and estimate its range, keeping it in the center of the 
sight. 

2. When the target is at 1,800 feet, fire a sighter burst (10 or 15 rounds), keep¬ 
ing the target in the center of the sight. Note the direction of trace and the end 
of the trace with reference to the ring. 







Figure 49. 



Figure 50. 


3. Put enemy where end of trace appears, or, better, at 1,500-foot mark, opening 
fire when the range is 1,500 feet. 

4. Hold enemy at same place in ring, firing at will until range is 1,000 feet. 
Then move him in on trace to the 1,000-foot point, approximately one-half (five- 
ninths) the distance from the sight center to the end of the trace. 

5. As the enemy closes in, keep him on the part of the trace corresponding to 
his range until he is at 500 feet. Then fire point blank. 

6. When the plane breaks away shift point of aim to the proper lead along the 
line of apparent motion. 

At the present time 1,800-foot trace does not have very wide dis¬ 
tribution among the United States Air forces. It is expected, how¬ 
ever, that it will not be long before it is available to all combat units. 





38 


Consideration has also been given to a trace which burns different 
colors at certain specified range intervals. 

The student should realize that it may be difficult to use the tracer 
method in a hand-held gun for the reason that it is hard to hold the 
weapon steady enough to determine the trace pattern. Ideally, it 
should be used in a power-operated turret. 

Most turrets have two guns mounted with a single sight. When 
using tracer with a twin installation the two traces appear to enter 
the ring from the side and converge at the center. If it is a no- 



FiGURE 51.—Photograph of tracer. 


allowance shot, the bullets appear to fly rapidly to the center and 
remain near it (fig. 45). When an allowance is required, their appar¬ 
ent positions are displaced from where they would have been in the 
no-allowance shot by distances which depend on the angular motion 
of the gun in following its target, and the resulting path of the trace 
is illustrated in figures 46-50. 

It is desirable that the sight axis be aligned parallel to the barrels 
of the guns. If it is not, the complex curvature of the apparent path 
due to the combined effects of the offset of the eye and the allowance 
conditions can be very confusing. 

It is difficult under some conditions to estimate the direction in which 
allowance should be made. Hence any device which indicates this is 
valuable. The ideal, of course, is the 1,800 foot trace which indicates 
not only direction but magnitude of allowance. 



39 


Figure 51 is a reproduction of an actual photograph of tracer. 
Notice particularly the apparently great length of the projectile just 
leaving the barrel. This is caused by the apparent rise of the bullet 
to the sight center, in this case the lens of the camera, which is posi¬ 
tioned some 2 feet above the gun. 

DANGER IN USE OF TRACER 

The most frequent and serious errors in the use of tracer occur 
through faulty range estimation. Bather than trust his calculation 



the gunner relies on what he sees of the trace and is misled into firing 
according to his optical illusion. 

Ordinarily an observer estimates the range of an object going away 
from him in one of two ways: 

{a) He may unconsciously compare the relative size of the object with 
that of some other object near it. Of course, an air gunner 
cannot use this method to estimate the distance of a tracer bullet, 
because he is shooting out into space where there are no land¬ 
marks along the path of the trace to the size of which it may be 
compared. 

(5) He-may compare the object’s present size with what it was when 
it was nearer to him. This is the method he would use in esti- 














































40 


mating distance down a railroad track. As the tracks appeal 
to converge, he uses the apparently narrowing distance between 
them to estimate the ranges or various points along the line. 
The same effect is produced when one uses a string of telephone 
posts. Standing at the first post, the observer judges his dis¬ 
tance from the last post by comparing its apparent size with the 
one beside which he is standing. 

Due to his long training at judging distance by this method, the 
gunner may unconsciously estimate range this way while using 

, Vi- 



tracer. The error is fatal, because the usual tracer does not burn 
at a constant intensity. It burns out as it goes along. The fact 
that the bullet diminishes in intensity while it is going away from 
the gunner tends to make the gunner overestimate his range. 

The gunner absolutely cannot rely on either his intuition or the 
trace to estimate range. He must use his ring and tracer method 
fii'st to get the target plane's range and then to position it properly 
on the trace. 


Section VIII. MACHINE GUN 


A gunner must understand his weapon. The free gunner fires the 
B(rowning) A(ircraft) M(achine) gun, caliber .30 and the BAM gun, 
caliber .50. The .50 BAM gun is coming into more and more gen¬ 
eral use in the fleet, but most instruction is carried on, for economy 
and for other reasons, with the .30 BAM gun. The operation of the 
two guns is essentially similar. The chief diflerences are easily 
explained. 

Construction and operation. 

The construction of the .30 BAM gun is explained in detail, with 
assembly diagrams in chapter II of G-1^ Aircraft Gunnery^ a ground- 
school manual issued in 1937 by the Aviation Training Department 
at the United States Naval Air Station, Pensacola. The operation 
of the gun is best learned in the stripping room under guidance. 

Nomenclature. 

The BAM .30 caliber M2 ^ flexible gun is made up of about 300 
parts. Those listed below are the parts the free gunner is expected 
to learn during his early study of the weapon. The parts marked 
with an asterisk are ordinarily removed in practice stri])ping. 


Nomenclature, Browning M-2, 30-Caliber Machine Gun 


GROUP 1. casing group 

1. Barrel jacket. 

2. Front barrel bearing. 

3. Front barrel bearing plug. 

4. Front mount adapters, left and right. 

5. Rear mount adapters, left and right. 

6. Trunnion block. 

7. Front cartridge stop.* 

8. Rear cartridge stop.* 

9. Belt holding pawl.* 

10. Belt holding pawl spring.* 

11. Belt holding pawl pins (2). 


GROUP 1. CASING GROUP-CON. 

12. Extractor cam. 

13. Extractor feed cam. 

14. Breech lock cam. - 

GROUP 2. BACK PLATE GROUP 

1. Buffer tube. 

2. Back plate latch spring and plunger. 

3. Buffer plate. 

4. Buffer adjusting screw. 

5. Trigger. 

6. Trigger safety,. 

7. Grips. 


^ All guns of the same caliber, but of a different design, are distinguished by diiferent 
“marks.” The first design built of any caliber is labeled Mark 1. As each new design 
appears, it is assigned a new mark ; e. g,, Mark 2, Mark 3, Mark 4. If the design of a 
gun is modified in a minor way, the gun retains the same mark, followed by a “modifica¬ 
tion number.” Thus a 14-inch, Mark 1, gun would, on being given a different lining, be 
designated as the 14-inch, Mark 1, Mod. 1 gmn. This system of designating by marks and 
modifications is applied to all ordnance units, such as mounts, breech mechanisms, firing 
locks, powder tanks, sights, and telescopes. Until recently the mark number was written 
with a roman numeral. (The M2 used in above designation is in reality an Army symbol 
which is similar to the Navy’s term “Mark 2.”) 

Airplane designations follow a similar system. PBY means P(atrol) B(omber) built by 
Consolidated (Y), A modification would be PBYl. A mark number for a new design by 
Consolidated would be PB2Y, and so on through PB2Y1, PB2Y2, PB3Y, PB4Y, PB4Y1, and 
PB5Y, for instance. 


( 41 ) 



42 


GROUP 3. COVER GROUP 


GROUP 5, LOCK FRAME GROUP—Continued 


1. Cover plate. 

2. Cover detent pawl and spring.* 

3. Cover latch. 

4. Cover lever. 

• 5. Cover latch spring. 

6. Cover extractor spring. 

7. Cover extractor cam. 

8. Belt feed lever. 

9. Belt feed lever plunger and spring. 

10. Belt feed lever pivot stud. 

11. Belt feed slide. 

12. Belt feed pawl. 

18. Belt feed pawl arm. 

14. Belt feed pawl pin and spring. 

GROUP 4. BARREL GROUP 

1. Barrel. 

2. Barrel locking spring. 

3. Barrel extension. 

4. Breech lock. 

5. Breech lock pin. 

6. Barrel plunger stud. 

GROUP 5. LOCK FRAME GROUP 

1. Lock fmines, left and right. 

2. Lock frame cams. 

3. Lo(*k frame guides, left and right. 

4. Lock frame retainer spring. 

5. Accelerator.* 

6. Accelerator pin.* 

7. Accelerator stop. 

8. Barrel plunger. 

9. Barrel plunger spring. 

10. Lock frame separator. 


11. Lock frame spacer. 

12. Trigger bar.* 

13. Trigger bar pin and spring.* 

GROUP 6. BOLT GROUP 

1. Alternate feed bolt. 

2. Alternate feed bolt switch. 

3. Alternate feed bolt switch, plunger, 

and spring.* 

4. Extractor. 

5. Extractor plunger and spring. 

6. Extractor stop. 

7. Ejector. 

8. Ejector pin and spring. 

9. T-slot. 

10. Recoil plate. 

11. Cocking lever.* 

12. Cocking lever pin.* 

13. Bolt handle.* 

14. Sear. 

15. Sear plunger and spring.* 

16. Sear holder.* 

17. Sear holder plunger and spring.* 

18. Sear stop and pin.* 

19. Firing pin. 

20. Firing pin spring. 

21. Firing pin striker. 

22. Driving spring.* 

23. Driving spring rod collar. 

24. Driving spring rod. 

25. Driving spring rod head. 

GROUP 7 

Retracting slide (free gun). 
Operating handle (fixed gun). 


Stripping. 

Disassembling the gun to find or repair a malfunction, or to study 
the construction is known as stripping it. Field stripping is dis¬ 
assembling the gun to the extent practicable for making repairs in 
action. Field stripping the .30 BAM gun consists of 13 steps. 

1. Raise the cover. 

2. Remove the back plate. 

3. Remove the bolt. 

4. Remove barrel and lock frame. 

5. Remove lock frame from barrel. 

6. Replace barrel and lock-frame group in casing. 


43 


7. Break down the bolt by removing: 

(а) Driving spring. 

(б) Extractor. 

( c ) Alternate feed bolt switch. 

( d) Alternate feed folt switch plunger and spring. 

(e) Cocking lever pin. 

if) Cocking lever. 

i9) Sear stop and pin. 

{h) Sear holder. 

(i) Sear holder plunger and spring. 

(;) Sear. 

(/c) Firing pin. 

8. Reassemble the bolt. 

9. Reassemble the gun. 

10. Close the cover. 

11. Charge the gun. 

12. Pull the trigger. 

13. Put the gun on safety. 

These steps should normally be completed in 2 to 3 minutes. 

Operation. 

In the Browning aircraft machine gun, caliber .30 M2, the force of 
recoil is used to perform the various mechanical operations of the gun. 
The operation of the various parts can best be understood by careful 
study of a mechanically operated cut-away gun. 

Upon the discharge of a cartridge, the recoil of the barrel is trans¬ 
mitted to the bolt and the barrel extension. Part of the remaining 
energy in the barrel extension is transmitted to the bolt by the acceler¬ 
ator, and the rest is dissipated in the back plate buffer by way of the 
lock frame. 

During the recoil the empty cartridge case has been removed from 
the chamber and a live round has been extracted from the ammunition 
belt and placed in position for chambering. 

Upon counterrecoil of the bolt, due to the energy stored in the driv¬ 
ing spring, the empty cartridge case is ejected and the chambering 
of the live round begins. 

The bolt trips the accelerator back into its original position, causing 
counterrecoil of the barrel and attached barrel extension. The bolt, 
momentarily delayed by tripping the accelerator, continues forward; 
the live cartridge is fully chambered with proper timing, and the 
breech lock falls, locking the bolt to the barrel extension. The gun 
is now ready to fire. 

Accelerator. 

The four functions of the accelerator are sometimes overlooked. 

1. In recoil it relays energy from the barrel extension to the bolt. 

493667*—43-4 



44 


The recoil of the barrel and barrel extension is thus slowed up while 
the recoil of the bolt is hastened. 

2. It effects this boost of the recoil energy of the bolt just as extrac¬ 
tion is occurring. 

3. It holds the barrel and barrel extension in the recoil position, 
with the barrel plunger spring compressed, thus preventing the barrel 
from returning to its forward position before the bolt has completed 
its recoil. 

4. In counterrecoil, it relays energy from the bolt to the barrel 
extension, moving the barrel forward ahead of the bolt so the breech 
lock in the barrel extension will rise at the proper time. 

Thus the accelerator, both in recoil and in counterrecoil, serves 
as an energy transmitter and as a timing regulator. 

Comparison of .30 and .50. 

Armor adequate to protect the vital spots of a plane against .30- 
caliber bullets can be added without too much increase in weight. To 
overcome such armor the .50-caliber ground gun has been redesigned 
for use in the air. In construction and operation it is, in general, simi¬ 
lar to the .30. Its principal differences, other than its greater size 
and weight, are the use of an especially strong double driving spring 
and of a special mechanism for controlling the rate of fire. 


.SO BAM gun compared with .50 BAM gun (free) 



.30 

.50 

Weight (pounds) .. 

About 20 
About 40 
M2 385 
2,700 

1,100-1, 200 

About 70 
About 54 
1,825 
2,700 

2 400-850 

Overall length (inches)_ _ 

Weight of one complete round (ball) (grains). . . 

Muzzle velocity Ml and M2 (ft./sec.)'_ 

Rate of fire (rds./min.)__ . 



' Muzzle velocity of M2 ball cartridges manufactured since February 1941 is about 2,800 feet per second. 

2 Variation in rate of fire depends on whether gun has double driving spring or not. The double spring 
will be found in most of the models now in use. 

Oil buffer. 

The oil buffer, which is used to control the rate of fire of the .50, 
takes the place of the lock frame group in the .30. 

The oil buffer controls the rate of fire by regulating the speed of 
recoil. When the adjusting key is turned all the way left it sets a 
valve to offer little resistance to the flow of oil backward, thus allow¬ 
ing the piston to recoil quickly, and leading to rapid fire. As the 
key is turned farther and farther right the valve offers more and 
more resistance to the flow of oil, allowing the piston to recoil less 
and less quickly and leading to slower and slower fire. 

The other differences between the .30 and the .50 are best discovered 
at the stripping table. 

















45 


O.B. PACKING GLAND SPRING- 
O.B. PACKING GLAND- 
O.B. PACKING GLAND RING 
aB. TUBE CAP. 


rPiSTON FILLER 
SCREWS 



BARREL EXT 
SHANK 

O.B. GUIDE 
O.B. SPRING 


RELIEF VALV 


SPRING 

PLUNGER 

CREW 


-O.B. 

TUBE 


OIL BUFFER TUBE ASSEMBLY 
(RECOIL POSITION) 

.50 CALIBER MACHINE GUN 

Figure 54.— Oil buffer. 

Care. 

Efficient performance can be expected from the .30 or .50 machine 
gun only if it is given proper care. It must be cleaned every time 
it is used; it must be kept oiled; and when not in use, it must be 
carefully stored. 

Cleaning. 

The bore and other parts must be cleaned, and accumulations of 
hard carbon removed from the front barrel bearing after each use 
of the gun, preferably immediately after firing, and in any event on 
the same day. 

The gun must be cleaned on 3 successive days after it has been 
fired, and inspected daily thereafter to avoid corrosion. 

Cleaning the bore: Materials. 

Cleaning the bore requires: (1) a bucket of hot water and issue 
soap or sal-soda (or, lacking soap and sal-soda, hot water alone); 
(2) sperm oil; (3) a cleaning rod; (4) dry, clean flannel patches, 
and (5) a brass or bronze wire brush fitted to screw into the end 
of the cleaning rod. 

Before the bore is cleaned, all groups are removed from the gun. 
Cleaning the bore: Procedure. 

1. Thread a flannel patch into the eye of the cleaning rod. 

2. Hold barrel, muzzle down, in the hot water. 

3. Insert rod with patch into breech, and plunge rod forward and 
back for about a minute, pumping water in and out of bore. 

4. With the bore still in the water, run brass or bronze brush for¬ 
ward and back through the barrel three or four times. 





























46 


5. Pump water through again. 

6. Take barrel out of water, and dry the cleaning rod. 

7. Swab bore thoroughly with dry, clean patches until it is per¬ 
fectly dry and clean. Use a flannel patch on a stick if necessary, to get 
chamber thoroughly dry and clean. 

8. Saturate a patch with sperm oil, and swab the bore and chamber 
with it, allowing a thin coat of oil to remain in the bore. 

Cleaning other groups. 

Using a small pointed instrument covered with a flannel patch to 
get dirt out of all recesses, clean and wipe all the other groups with 
a dry rag, taking care to remove dirt from the belt holding pawl. Then 
wipe all parts with an oily rag. 

Removing carbon. 

The carbon which accumulates in the front barrel bearing may bind 
the barrel and prevent the gun from firing. 

1. Remove the front barrel bearing plug by camming its crimped 
flange out of the recess in the front barrel bearing with a wrench. 

2. Scrape out the carbon with a scraper or reamer. 

Oiling. 

All bearing parts must be oiled before the gun is fired. But oil 
must be used sparingly, because low temperatures encountered in 
flight will lead to thickening of the oil and malfunctioning of the 
gun. 

Particular care must be taken to oil: 

(1) The exterior of the barrel at the breech end. 

(2) The cover extractor spring. 

(3) The cover extractor cam. 

(4) The cover detent pawl. 

(5) The cocking lever. 

(6) The groove in the bolt for the belt feed lever. 

(7) The grooves in the barrel extension for the bolt ribs. 

(8) The breech lock cam. 

(9) The extractor cam. 

(10) The sear mechanism. 

(11) The ways of the belt feed slide. 

When guns are fired on the ground at temperatures of 45° F. or 
higher, sperm oil. United States Navy Specification 14-6-5 (latest 
issue), should be used. In an emergency, motor oil, S. A. E. 10 or 
♦its equivalent may be substituted. 

When guns are fired on the ground at temperatures below 45° F., 
or in the air regardless of temperature, lubricating oil for aircraft 
instruments and machine guns. United States Army Specification 
2-27 (latest issue), should be used. In an emergency, transformer 
oil. United States Navy Specification 14-0-12 (latest issue) should be 
used. 


47 


The term ‘‘fired on the ground” applies to guns on ground train¬ 
ing ranges. It is not necessary to change to ground type oil for 
occasional firing when confined to short bursts to check functioning 
of the gun, and when boresighting. 

For oil buffers on the caliber .50, United States Army Specifica¬ 
tion 2-27 should be used under all service conditions. United States 
Navy 14-0-12 transformer oil is too thin to produce sufficient buffing 
action. 

Parts are best oiled with clean, grit free, lintless cloth, and for 
operation between 45° F. and 0° F., a slightly oiled cloth should be 
used. Below 0° F., parts should be completely wiped off with a 
dry cloth after oiling. In extreme temperatures, even this method 
has been known to permit enough oil to remain to congeal and to 
freeze the gun. In these cases, kerosene may be applied sparingly 
to the internal mechanism without removing the gun from the gun 
casting. Use of kerosene requires close daily inspection, and if rust¬ 
ing occurs, the gun must be disassembled, rust removed, and pro¬ 
tecting oil applied. 

The ammunition and links should be clean and free from oil and 
not wiped with an oily rag. 

United States Army Specification 2-7 is not a rust preventative. 
In humid atmosphere, metal surfaces covered with it will rust rapidly. 
When guns are operated regularly, it shall be used, application being 
made frequently. As an exception, sperm oil shall always be used for 
preserving the chamber and bore. This should be applied from the 
breech end using a patch on a cleaning rod. 

If weapons are left more than 48 hours without flying, they should 
be disassembled and oiled with sperm oil or S. A. E. 10, and reoiled at 
7-day intervals. 

Flight care. 

To insure effective operation in practice or conflict, and to avoid 
accidents on the landing field certain precautions are necessary. 

BEFORE FLIGHT 

1. Check to see that the bore is thoroughly clean and free from 
grease. 

2. Tighten all screws. 

3. Test functioning by hand. 

{a) In particular, check firing pin in first position. 

(&) Pass dummies through gun. 

(c) Check operation of slide and cables. (This is for a fixed gun.) 

4. Check oil. Do not use too much oil. If going into high altitudes, 
be especially sparing, and use appropriate special oil. Oil congeals 
with low temperatures and tends to slow action of gun. 


48 


5. Check that sights are firmly fixed in position. With a reflector 
sight check the continuity of the electric circuit, and be sure there is 
an adequate supply of good spare bulbs. 

6. Check belt for corrosion, or other flaws. 

7. Take along kit for emergency repairs, including in particular a 
complete bolt. 

AFTER FLIGHT 

1. Before landing 

(а) Unload gun completely. 

(б) Lift cover and pull back bolt to see that no cartridge is in chamber. 

(c) Remove belt from container. 

2. After landing 

(а) Clean bore and all working parts. If impossible, give temporary 

coating of oil. 

(б) Release firing pin spring. (Weak firing pin spring leads to denting 

primers without exploding them.) 

(c) At first opportunity, dismount gun and clean it thoroughly. 

Storage. 

Storage involves special procedures both wdien the gun is put away 
and when it is taken out. 

PREPARATION FOR STORAGE 

1. Clean the gun thoroughly. 

2. Wipe all parts perfectly dry with rags. (In damp climates 
special care must be taken that rags are thoroughly dry.) 

3. After a part has been dried handle it with an oily rag. Do not 
touch it with bare hand. 

4. Coat each metal part with rust-preventive compound. 

5. Assemble the gun, with bolt forward and firing pin released. 

6. Set the gun in the packing case and close the case carefully. 

REMOVAL FROM STORAGE 

All traces of rust-preventive compound must be removed from guns 
taken out of storage, especially from all recesses in wdiich plungers 
operate. This is done by applying dry-cleaning solvent (a noninflam¬ 
mable and noncorrosive petroleum distillate) with swabs to large parts 
and using it as a bath for small parts. 

The dry-cleaning solvent is completely removed from all parts by 
wiping them with light-colored cloths until no staining of the cloth 
occurs. 

It is important to leave no finger marks, because they induce cor¬ 
rosion. 

After alT surfaces have been thoroughly cleaned they should be 
protected with a thin film of lubricating oil, applied with a rag. 


49 


Eliminating stoppages. 

The Browning aircraft machine gun is a reliable gun. Stoppages 
have become more and more rare, especially since in recent years all 
ammunition has been more uniform and of higher quality. And air¬ 
craft ammunition is of the highest quality manufactured. Because 
the best and newest equipment of all sorts is sent to the fleet, rather 
than to training stations, however, and because a sustained effort is 
made to train gunners to cope with many kinds of stoppage, the be- 


POSITION 2‘'POSITIOK IMPOSITION 



3MP05ITIOH 2‘‘P05ITI0HrP05ITION 




Figure 55, 


ginner sometimes suspects that the gun is not reliable. This suspicion 
is ill-founded. 

Stoppage of the gun in action is highly infrequent. Time spent on 
the long list of stoppages that follows is to be thought of as insurance. 
None of these stoppages is likely, but when one does occur in action 
it is healthy to be able to handle it. 

By stoppage is meant any accidental cessation of fire, except one 
due to exhaustion of ammunition. It might be due, for instance, to 
corroded feedbelt links. 

A stoppage due to improper action of a part of the gun is called a 
malfunction. 

Stoppages which can be corrected in flight by an automatic routine 
called immediate action^ or by a minor adjustment, are called tem¬ 
porary., as distinguished from prolonged stoppages. 


























































50 


Stoppages are further classified, as a first step in determining how 
to cure them, by the position of the bolt, which can be told from the 
position of the charging handle. (See fig. 55.) 

A stoppage is classified as:’ 

1st position: Wlien the bolt stops all way forward in the battery. 

2d position: When the bolt stops just out of battery, or back farther as 
much as half way. 

3d position: When the bolt stops half way or farther back. 

It is helpful to observe, by testing manually which way the bolt 
will move, whether a 2d position stoppage occurred in recoil or 
counter recoil. 

Checking through a table of stoppages grouped by position helps a 
gunner to master his gun. This is best done in the stripping room 
with a gun at hand. 

The best way to eliminate stoppages remains, or course, prevention 
rather than cure. Stoppages are prevented, (1), by proper care of 
tlie gun (see p. 64), and (2), by careful calibration of ammunition. 
(See section on Ammunition.) 

STOPPAGES 

First position 

Cause Hotc to discover 

A. Cartridge defective; 

1. Primer defective- Dent on cap, no lire. 

2. Short round-Round not extracted from belt. 

B. Feeding defective: 

1. Belt loaded improperly_ 

2. Pawls or pawl springs defective_ 

{a) Feed- Round not extracted from belt. 

(&) Holding- Belt slides out of feedway. 

3. Belt feed lever defective_ 

4. Bolt switch defective_No feed. 

5. Extractor defective_ Do. 

6. Cover extractor spring_Do. 

C. Firing Mechanism defective: 

1. Firing pin short or broken-No fire; dent in primer slight or 

lacking. 

2. Firing pin spring weak-Same as above. 

3. Cocking lever worn or bent-Shot in receiver or misfire. 

4. Trigger bar bent: 

{a) upward-Runaway gun. 

(?;) downward_No fire. 

5. Trigger tip broken_Do. 

6. Sear notch engagement faulty_ Runaway gun. 

7. Sear holding spring broken-Gun fires once or twice; stops. 



















51 


Cause 

A. Cartridge defective: 

1. Round bulged 


Second position 

Hotc to discover 

-Slow action ; failure to seat in 

chamber. 


2. Bullet loose_ 

3. Links tight- Slow action. 

B. Cases separated: 

1. Which stay in chamber when bolt 

pulled to rear_ 

2. Which are pulled out of chamber by 

new round_ 

C. Headspace faulty; 

1. Too loose-Ruptured cartridge. 

2. Too tight- Gun will not fire. 

D. Barrel-locking spring broken-Head si)ace becomes too tight or 

too loose. 


E. Extractor or ejector defective_ 

F. Driving spring weak- Slow action or failure. 

G. Barrel plunger spring weak-No pre.ssure on accelerator. 

H. T-lug broken_ 

I. Carbon-Barrel binds. 


How to discover 


Third position 

Cause 

A. Cartridge defective: 

1. Rim thick or battered_ 

2. Rim thin- Bullet strikes face nf barrel. 

3. Cannelure broken-!_No extraction. 

B. Recoil-plate defective_ 

C. Feeding-system defective: 

1. Belt-feed pawl arm bent or broken Double feeding possible. 

2. Cover extractor cam defective, worn.. 

3. Extractor worn (top cam)_ 

4. Extractor spring weak, or improp¬ 

erly seated_ 

5. Ejectoi* .spring solid height too 

great_ 

6. Ejector spring weak_ 

D. T-slot worn or broken_Bullet strikes face of barrel. 


^Bullet strikes face of barrel. 


Other stoppages. 

Ill addition to these stoppages, broken breech lock depressors slow 
up the action of the gun slightly, and worn lands and grooves allow 
some of the gas of the explosion to escape, causing slowing of the 
action of the gun with loss of range and accuracy. Also bullets yaw 
excessively. 

Immediate action. 

Whenever a gun jams, the gunner responds automatically with a 
routine calculated to eliminate the stoppage at once. Tliis routine is 
called immediate action. Its purpose is to eliminate rather than to 


























52 


diagnose the stoppage. Immediate action is used in a strict and in an 
extended sense. 

Strictly, immediate action consists of: 

1. Tapping the cover. 

2. Straightening the belt. 

3. Charging the gun. 

This is done without taking the eye from the sights. Charging is 
pulling the bolt handle smartly back and releasing it quickly as soon 
as it is all the way back. To avoid injury, it is important to observe 
carefully the technique of charging as demonstrated in the classroom. 
Failure to release the handle—‘Tiding the gun”—damages the driving 
spring and reduces the efficiency of the gun. 

Immediate action, in the extended sense, covers two or three further 
steps. This routine, as analyzed below, must become automatic. The 
gunner should early learn it, understand it, and practice it. 

The steps indicated are the logical ones taken by gunners in order 
to locate the cause of the stoppage. Most of the stoppages mentioned 
will become readily apparent if this procedure is carried out. 

CHART OF IMMEDIATE ACTION 
I. Gun fails 


II. 1. Tap cover; ( 2 ) straighten belt; (3) charge; 

4. Notice if belt feeds; and 

5. Feel with left hand under receiver for ejected round. 


III. Bound ejected. 

IV. Press trigger. 


III. Bound NOT ejected. 

I^. Baise cover and remove first 
round from belt. 


V. Gun still fails. 


V. Cartridge in gun. 


V. Cartridge NOT in 


gun. 

VI. Change bolt.^ VI. Bemove cartridge. (VI.) 

i 

VII. 1 . Beload. 

-^ 2 . Beaim. 4 ._ _ 

3. Fire. 


iThe steps taken up to this point have narrowed down the cause of failure to the bolt 
group. Since there is no time in combat to analyze the trouble further, the gunner replaces 
the faulty bolt with a sound one from his kit. ^ replaces 















Section IX. AMMUNITION 


The gunner should know how to identify and care for the types 
of ammunition ordinarily used in aerial firing. 

Types. 

Five principal types of ammunition are used in the machine gun: 

1. Ball: for use against personnel and light material targets. 

2. Armor-piercing: for use against armor, concrete, and the like. 

3. Tracer: for checking range and aim, and for setting fires. 

4. Incendiary: for setting fires. 

5. Dummy: for training. 

An explosive bullet, called the De Wilde bullet, has also been devel¬ 
oped for use in machine guns. 


Service (Baix) Ammunition 

General. 

When the cartridge is fired, the following procedure takes place all 
in a split second: The gun’s firing pin hits the primer cap, setting off 


CAf^Tf^fDQC CASE- 

cannelure of case 

I 

HEAD I VENT 


pCUPf^O-N»CI\EL jACI(eT 

lead antimony 

ball 


PRIME Ft* 


U 


0 / 


PRIMER POC»(ET 
PRopELXANT CHA^Oe 



SHOULDER \ ^MOUTH 

canneloi^c of Bullet 


Figure 56. — Ball cartridge. 


the explosive charge, usually mercury fulminate, in the primer pocket. 
A strong flash enters the propellant charge, ordinarily nitrocellulose, 
and sets up controlled progressive burning from the aft to the forward 
end of the cartridge. As the pressure changes, the bullet leaves the 
mouth of the cartridge case to which it has been secured by three 
indents and coning. The main core of the bullet is lead and antimony, 
and the jacket is cupro-nickel. (A tin-coated gilding metal jacket is 
now used owing to shortage of cupro-nickel.) 

As pressure keeps building up behind, the bullet picks up speed in 
its course down the barrel. The powder is specially treated so that it 
burns, not in one suddent blast which would raise terrific chamber pres- 

( 53 ) 













54 


sure, but steadily and progressively so that it is practically expended 
at the moment the bullet emerges from the barrel. If all the powder 
has not been consumed at this jDoint, there will be excessive muzzle 
blast and a consequent secondary recoil. 

When the bullet strikes its target, the soft cupro-nickel jacket imme¬ 
diately smears on the target’s surface, giving the core of the bullet an 



20 MM. 

^ .7^7 IMCHE$ 




Figure 57. 



initial grip. The bullet can then pierce and screw its way through a 
hard surface which would otherwise turn it aside. 

Although the only .30-caliber ball cartridge now being manufac¬ 
tured is the M2, some Ml ammunition is still in use. The differences 
between these two types of ammunition can best be understood by con¬ 
sidering the circumstances of their development. 

The lower the chamber pressure by which a satisfactory high muzzle 
velocity can be attained the better. The higher the chamber pres¬ 
sure the tighter the cartridge cases are jammed against the chamber 






























55 


walls, with consequent difficulty in extracting them, and also the 
greater shock both to gun and gunner. 

1903 and 1906. 

When the 1903 model Springfield was adopted by the Army, it 
fired a .30-calibre, 220-grain, round-nosed, flat-based cartridge with 
a muzzle velocity of about 2,200 feet per second. Three years later, 
in 1906, the cartridge was redesigned with a pointed nose and flat 
base. Its weight was reduced to 150 grains and its muzzle velocity 
increased to 2,700 feet per second. It developed a chamber pressure 
of 50,000 pounds per square inch. In this design it remained sub¬ 
stantially unchanged until World War I. 

Ml. 

Though our rifle and machine-gun fire was ineffective beyond 
5,100 feet, we found that the Germans were able to cause casualties 
with a 7.9-mm. boat-tailed bullet at 15,000 feet. Experiments per¬ 
formed by Capt. E. C. Crossman at Daytona Beach led to the issue, 

FLAT BASE | ^ 1906 

BOAT tail I N ivi.| 

ADDITION ^_X ‘ * *• 

Figure 58. —Boat-tail. 

ill 1926, of the cartridge known as Ml. It was exactly the same shape 
as the 1906 bullet, except that it was lengthened to form a 9° boat- 
tail. This increased the weight by 22 grains, making 172 grains in all. 

The muzzle velocity was 2,700 feet per second. The powder in the 
cartridge was improved so that it burned more steadily and slowly 
and imparted a ptish rather than a blow to the bullet. 


Comparison of 1906, Ml and M2 ammunition 



1906 

Ml 

M21 

Shape of base___. . 

Flat 
2,700 
150 
50,000 
1,536 
1,800 
5,100 
10,000 

Boat-tail 
2,700 
172 
47.000 
1,634 
3,000 
9,000 
16, 500 

Flat 
2,700 
150 
39,000 
1,536 
1,800 
5,100 
10,000 

Muzzle velocity (ft-/sec.)--- 

Weight of bullet (grains)--- 

Chamber pressure (Ib./sq. in.) _ _ 

Remaining velocity at 1,800 feet (ft./sec.)- 

Effective accuracy (feet) (from ground)... -- 

Effective range (feet) (from ground). --- 

Extreme range (when barrel is elevated 30°) (feet) (from ground).. 


I Since February 1941, the muzzle velocity of the M2 ammunition has been raised to about 2,800 feet per 
second. 


M2. 

In the 1930’s, to meet the need for more fire power, the Garand 
rifle was adopted. Originally the Garand was designed for 0.276- 
calibre cartridges. Because they were smaller and lighter, a soldier 




















56 


could carry a larger number of rounds. Later when the rifle was 
redesigned to take .30-calibre cartridges, the Ml proved too pow¬ 
erful. 

The M2 was developed expressly to provide a cartridge suitable 
for the Garand. It is a flat-base bullet with the same primer and 
brass case as the 1906 type, and it weighs 150 grains. Except for 
its powder charge, which creates a smaller breech pressure—39,000 
pounds per square inch—the M2 is for all practical purposes the 
same as the 1906; it has the same exterior ballistics. 


Range 

1,800 feet 

2,400 feet 

3,000 feet 

3,600 feet 

Ammunition cal. .30. . 

Ml 

1 M2 

Ml 

1 M2 

Ml 

1 M2 

Ml 

1 M2 

Time of flight (seconds)___ 

0. 875 

0.887 

1.275 

1.320 

1.745 

1.838 

2. 285 

2. 430 

Remaining velocity (ft.-sec.)_ 

1,634 

1,536 

1,382 

1,260 

1,185 

1,075 

1, 051 

965 

Remaining energy (ft.-lb.) . 

1,020 

786 

730 

529 

536 

385 

422 

310 


1 The figures above are for M2 (1939). 

Comparison of Ml and M2. 

Up to 1,800 feet the flight characteristics of Ml and M2 (1939) 
are almost identical. Beyond 1,800 feet, however. Ml bullets are 
more accurate and hit with more striking and penetrating power. 
The boat-tail shape of the bullet and its added weight enable it to 
retain its velocity longer and make it less subject to ballistic forces. 

The Ml bullet, however, has its disadvantages. Its boat-tail end 
(a) makes it more difficult to manufacture with precision (5) causes 
more erosion at the breech (a) at times permits an unequal appli¬ 
cation of gas at the moment of ignition and consequent unequal gas 
pressure on the projectile as it passes through the bore. The wear 
and tear the Ml bullet causes in a machine gun which has a large 
bolt group to absorb the extra chamber pressure, is not such a 
serious problem. But since up to now it has proved more feasible 
to produce common ammunition for both machine guns and rifles, 
]\I2 has been in more regular use. 

In 1941, a new M2 cartridge was approved by the United States 
Ordnance Department which causes a chamber pressure of about 
45,000 pounds but raises the muzzle velocity to 2,800-2,900 feet per 
second. Out to 2.000 feet its trajectory is flatter and its penetrating 
power greater. In aerial fire, the time of flight is reduced. Muzzle 
blast and barrel erosion are also cut down considerably. 

Armor Piercing 

1. Oonsfmctzon.—Armor-piercing ammunition has the same case, 
primer, and powder charge as service ammunition. The projectile, 
however, has a hard-steel core cased within a lead and antimony sleeve 
and a steel envelope, coated with cupro-nickel. 

















57 


CUPRO-NICKEL JACICET 

steel envelope—1 

LEAD ANTIMONY SLEEVE — 

Steel slug—i 


PROPELLANT CHARGE 


cartridge case 



Figure '59.—Armor-piercing cartridge. 


PROJECTILE 


Weight of cartridge complete_ 389 grains 

Weight of projectile_ 150 grains 

Weight of powder charge_ 47 grains 

Length over all_ 3.329 inches 

Length of projectile_ 1. 258 inches 

Diameter of projectile_^ 0. 308 inch 


1 All .30-cal. ammunition is manufactured O.OOS inch oversize so that tlie jacket will tit 
into the grooves of the barrel. 

2. Trajectory ,—The trajectory is practically the same as that of 
ball ammunition up to 1,200 feet, and is slightly better (straighter) 
beyond that distance. 

3. Penetration .—The penetrative power of armor-piercing bullets 
is considerably greater than that of ball ammunition against any solid 
material. Their use is imperative in sectors where enemy planes 
are heavily armored. 

4. Effect on gun .—When this ammunition is used in quantity, the 
efficient life of a gun barrel is reduced from about 15,000 rounds to 
about 2,000 rounds owing to the excessive friction caused by the 
contact of the steel envelopes of the bullet against the bore. 


SOLDER 


LOW FUSIBLE ALLOY 



Incendiary 


1. Construction .—The incendiary cartridge has the same case, 
primer, and ^^owder charge as the service cartridge. The bullet has a 
cupro-nickel jacket with the nose slightly flattened. Extending from 
the nose half way to the rear of the projectile is the incendiary com¬ 
position, yellow phosphorus. As the bullet passes through the barrel 


























58 


the friction raised melts the plugs of low fusible alloy leaving two 
holes in the jacket. Oxygen then enters and ignites the phosphorus. 


Weight of cartridge complete_381 grains. 

Weight of projectile_147 grains. 

Weight of powder charge_:_45 grains. 

Length over all_ 3.329 inches. 

Length of projectile_ 1.426 inches. 

Diameter of projectile_ 0.308 inches. 


2. Trajectory ,—Because the incendiary bullet loses weight in the 
course of its flight, its trajectory varies from that of the service bullet. 

3. Effects .—Incendiary ammunition is designed for the purpose of 
igniting explosive or highly inflammable mixtures. Balloons and 
gasoline tanks are vulnerable targets. In addition to doing its pri¬ 
mary job it has an incidental tracer effect. By day it produces a 1-inch 
smoke trace and by night a distinct, but not blinding yellowish trace 
over a range of approximately 300 feet. 

There have been reports of muzzle burst taking place when using 
incendiary ammunition, but experiments have shown that these occur 
rarely, and when they do, that they are not harmful. 



CARTRIDGE CASE LEAD ^ ANTIMONY 

Figure 61.—Tracer bullet. 


BARIUM PEROXIDE 


JACKET 


Tracee 

1. Construction .—Tracer ammunition has the same case, primer, 
and powder charge as service ammunition. Unlike the incendiary, 
the tracer composition is ignited by the explosion of the powder 
charge and not by the friction of the bullet as it passes through the 
barrel. The tracer composition itself is pressed in the base of the 
bullet and positioned by a small washer. 

Weight of cartridge complete_ 385 grains. 

Weight of projectile- 150 grains. 

Weight of powder charge_ 44 grains. 

Length over all-3.329 inches. 

Length of projectile- 1. 449 inches. 

Diameter of projectile_ 0. 308 inch. 

2. Trajectory .—opinion of authoritative individuals to the 
contrary, the tracer bullet proves in practice to take the same trajec¬ 
tory as does the service bullet. 
















AMMUNITION 


MARKINGS 






s 

ni 



— ^ 


.30 CAL. 



■50 CAL. 


■ 





.30 AND .50 CALIBER AMMUNITION 
MARKINGS 


TYPE 

AMMUNITION 


BOX 

MARKINGS 


PROJECTILE 

MARKINGS 


BALL 


4" RED 


PLAIN COPPER 
COLORED 


TRACER 


2" GREEN 
ON YELLOW 


I I 


RED 




ARMOR 

PIERCING 


2" BLUE [_| 
ON 4" YELLOW 


BLACK 


ft 


INCENDIARY 


BLUE- 
INCENDIARY 
IN RED LETTERS 




BLUE 


DUMMY 


4" GREEN 


PLAIN 


BLANK 

(.30 CAL. ONLY) 


4" BLUE 


(PAPER WAD 
PROJECTILE) 


LINKED BALL 
AND TRACER 


2" YELLOW 
2" RED 
2" GREEN 


m 











































59 


3. Effects .—The tracer bullet goes just as far and hits with as much 
accuracy as the service bullet. (But its primary purpose is to help the 
gunner solve his sighting problem.) The trace which will probably 
be standard is one which burns at a constant intensity out to 1,800 
feet and then disappears. Colored or white, it is visible by day, yet 
not so strong that it is blinding at night. Consideration is being given 
to a trace which will burn so that it changes colors at specified ranges. 
At the present time, however, a variety of trace is in use, much of it 
too long-burning to be of practical use to the free gunner. 

Identification. 

The principal types of .30- and .50-caliber BAM gun ammunition 
are identified by colored markings on cartridges and on cartons and 
cases. The colored bands on the cartons and cases are vertical for 
.30 caliber, diagonal for .50 caliber. (See color chart, facing p. 60.) 

Packing. 

All small-arms ammunition used in naval aviation comes packed 
in metal-lined wooden cases of uniform size. Within the cases, caliber 
.30 cartridges come either in cartons (20 to the carton, 75 cartons to 
the case, making 1,500 rounds per case) or in clif s and handoleers (5 
to the clip, 12 clips to the bandoleer, 20 bandoleers to the case, making 
1,200 rounds per case). Within the cases, caliber .50 cartridges are 
packed only in cartons (10 to the carton, 28 cartons to the case, making 
280 rounds per case). 

Model and lot number. 

Each model of each type of ammunition is assigned a model number. 
And when ammunition is manufactured each lot is assigned a number. 
This lot number is marked on all packing containers and on the 
identification card enclosed in each container. It is required for all 
purposes of record, including reports on condition, functioning, and 
accidents. 

Grading. 

From time to time circular letters revising previous grading of 
ammunition are sent out from the Bureau of Ordnance to all ships 
and stations. Beside the number of each lot not known to be ex¬ 
hausted is listed a symbol which indicates the use or uses for w^hich 
that lot is deemed suitable. 

Symbols used in grading 


XC _ Primarily for machine guns (all types). 

AC(K)_ Primarily for synchronized machine guns; also 

suitable for rifles, automatic rifles, and other 
types of machine gun. 


493667®—43-5 




60 


R(AC)_ Primarily for rifles and automatic rifles; also 

suitable for all types of machine gun. 

mg _ Suitable for all types of machine gun except 

synchronized aircraft guns. 

K_Primarily for rifles and automatic rifles; also 

suitable for all types of machine gun except 
synchronized aircraft guns. 

1 _ For .45 pistols and submachine guns. 

2 _ For target practice ONLY with .45 pistols. 


8_ NOT TO BE USED. 

* _Opposite the grade indicates the lot is to be given 

priority in use. (First priority is always 
given to ammunition in opened containers. In 
grading ammunition the attempt is made to 
clear out old and small lots.) 

# _By the lot number indicates that ammunition is 

packed either in clips and bandoleers or in clips 
and cartons. All other ammunition is packed 
in cartons without clips. 

Labeling. 

On the labels of all cartons, on all packing containers, and on card 
sealed inside the metal liner at the top of each ammunition box the 
ammunition is identified as to: 

1 . Quantity. 

2. Type. 

3. Caliber. 

4. Model number. 

5. Manufacturer. 

6. Lot number. 

Care. 

Ammunition should be kept in boxes and stored in appropriate store¬ 
houses where it will be free from moisture, dust, and oil. Under no 
circumstances should it be taken into an armory or building where 
guns are stored, being cleaned, or being used for instructional purposes. 
With tracer and incendiary ammunition it is necessary to take special 
precautionary measures. 

While being transported, ammunition should never be subjected to 
rough usage lest the cartridges become distorted, causing undue wear, 
stoppages, and breakages in the guns. With belted ammunition special 
care should be taken. It should be transported to aircraft for rearm- 
ing in suitable boxes only. Incorrect handling causes unnecessary 
stoppages. When belts are carried bandoleer fashion, wrapped around 
the body, lounds woik loose and links become twisted, thus increasing 
the incidence of stoppages. 










61 

Six brief rules form the basis of proper care, handling, and preserva¬ 
tion of ammunition. 

1 . Open airtight container only when ammunition is needed for use. 
(Especially in damp climates, ammunition may corrode.) 

2 . Protect ammunition from mud, sand, dirt, water. Wipe off dirt 
and moisture at once. Wipe off verdigris or light corrosion. 

3. Use no oil or grease on cartridges. 

4. Fire no cartridges with loose bullets or with other defects. 

5. Expose ammunition to the direct rays of the sun for as short a 
time as possible. 

6 . Tag belts with the lot number of cartridges loaded into them. 
(Tagging is necessary in order to preserve the grade of ammunition.) 

Testing. 

A gunner who cares for his own life and the safety of his plane 
always carefully calibrates his ammunition before he takes off. When 
the mechanism of a machine gun is functioning properly, it is unlikely 
that there will be stoppages or jams, except those caused by defective 
ammunition. As a rule, a machine gun is very sensitive to defective 
ammunition. Thus, it is absolutely essential that the ammunition be 
faultless. The following tests may be given to ammunition to be used 
in guns during flight. 

Thick rims .—Pass the rounds through the T slot of a bolt. As the 
rounds are passed through, twist them. Condemn any rounds which 
bind or stick. 

Cartridge length .—With an armorer’s dummy set a gage at correct 
cartridge length. Condemn any rounds that are either too long or 
too short. 

Diameter .—Drop each round in the chamber of a new barrel. Con¬ 
demn any round which fits too tightly or loosely. 

Deep set or protrnding primer .—Pass a straight-edge over the head 
of each round. Condemn all rounds in which the primer is not flush 
with the head of the cartridge. 

Split case .—Examine the case for splits or any faults in the metal. 
If faults are detected, condemn the round. 

Loose hullet .—Try to pull the bullet out of the case with the fingers. 
If it is loose, condemn the round. 

Correct powder charge .—Shake the cartridge close to the ear and 
judge the charge by the sound. The powder should be loose enough 
to make a slight noise when shaken. If the charge is too large or too 
small, condemn the round. If balance scales are available, weigh the 
rounds against those known to be perfect. Ammunition with minor 
defects may be used for range work or for gun testing in some cases. 

Cartridge gages.— Cartridge gages are furnished for gaging air¬ 
craft ammunition before it is loaded into machine gun belts or maga- 


62 


zines. Every round should be gaged for over-all length and diameter 
and depth of primer. Any rounds which do not check within the 
limits should be discarded. 

The .30-caliber cartridge gage consists of three separate gages. To 
test a cartridge by this gage, the cartridge is breeched in gage No. 2 
and the fit is examined. Gage No. 1 is then pressed against the base 
of the cartridge, to check its length and diameter. Gage No. 3 is then 
applied to the base of the cartridge to check the depth of the primer. 


Section X. BULLET PATTERN 


Definition. 

Bullet 'pattern is the area covered by a large number of rounds fired 
by the same gun under constant conditions. 

Gone of fire sometimes used synonymously with bullet pattern^ is 
three dimensional and more properly refers to the portion of space 
which contains the trajectories of all bullets fired. 

Pattern is not complete unless a sufficiently large number of rounds 
are fired in determining it. Any variations in the gun, the mount or 
in internal conditions, result in new and different patterns. 

Bullets fired from a gun perfectly boresighted and perfectly aimed 
will not all strike the same point. No matter how rigidly a gun is 



mounted, if shots are fired in bursts, the bullets never leave the muzzle 
in a perfect stream, but spread out to form a slender, ever increasing 
cone of fire. This “spray effect” of the gun, which produces the cone 
of fire, is due to a number of factors outlined below. It is of primary 
importance in air free gunnery due to the fact that the recoil oper¬ 
ated machine guns used fire at a very rapid rate (1,200 rounds per 
minute), while installed in very flexible mounts. Although the .50- 
caliber machine gun produces a smaller pattern than does the .30- 
caliber gun, the difference is so slight that conclusions reached below 
may be considered as applicable to both weapons. Only in power 
turrets is the pattern area reduced to any appreciable extent. 

The pattern formed will approach a circular or elliptical shape 
with the point on which the gun has been boresighted as its center. 

( 63 ) 













64 


Causes of bullet pattern. 

1. Barrel whip. — {a) All guns have a certain amount of play in the 
barrel. During firing that movement may be in any direction from 
the original line of fire, causing succeeding shots to take different 
lines of flight. 

2. Recoiling parts. — (a) As parts within it recoil and counter¬ 
recoil, the entire gun itself moves. 

3. Type and amount of explosive charge. — {a) Since the explosive 
charges in rounds of ammunition varies slightly, a gun’s muzzle 
velocity changes. As the muzzle velocity varies, so do the paths of 
projectiles it fires. Some newer types of explosive charges result 
in smaller bullet patterns than the older, less stable types. Today 
aviation ammunition is so well standardized that it is only a minor 
contributing factor in bullet pattern. 

4. Type of arrurmmition .— 

{a) The flight characteristics of some types are basically better than 
those of others. 

(b) Variations in weight and shape of the projectile result in 
less accurate firing and larger patterns. 

(1) Tracer and incendiary bullets change weight and shape 
while in flight as their tracer or incendiary compound is 
burned up. 

5. Movement of the mount .— 

(a) Even the most rigid mount permits some movement of the gun 
and consequent bullet dispersion. 

{h) Because they are less rigidly mounted, flexible guns produce 
considerably larger patterns than do fixed guns. 

(<?) The C16 gun mount adapter results in a markedly reduced 
bullet pattern owing to a recoil mechanism being incorporated in the 
mount. 

6. Variable factors due to mechanical shortcomings .— 

{a) Imperfections in the barrel. 

(b) Variations in the speed of recoil. 

Patterns at different ranges. 

1. The size of a pattern varies slightly more than proportionally 
with the range. (For convenience, however, diagrams shown are 
based on the assumption that the increase in pattern is exactly pro¬ 
portional to the increase in range.) 

{a) At 1,200 feet the diameter of the pattern of a .30-caliber M2 
machine gun, flexibly mounted but with grips held absolutely steady 
in a vise, is 20 feet. 

{b) At 2,400 feet the diameter of the pattern is more than 40 feet. 

2. The density of bullets within the pattern decreases as the dis¬ 
tance from the center of the pattern increases. For example, if a 


65 


pattern is 20 feet in diameter, the density 5 feet from the center is 
greater than that 7 feet from the center. Actually at 1,200 feet 
50 percent of the fire falls within an area 12 feet in diameter; the 
remaining 50 percent causes the pattern to be enlarged to a size 
20 feet in diameter. 

3. Data concerning the exact size of patterns of different guns at 
various ranges were not readily available when the manual was 
published but it is possible to allow for bullet pattern by means of 
reasonable approximations. 

Importance to the gunner. 

The farther away a target is, the greater will be the bullet scatter 
and the thinner the density of the bullet pattern. It is entirely 
possible that a gunner’s target may be in the center of his pattern 
and yet not receive a hit. Thus bullet pattern is one of the main 
factors limiting the effective range of aerial machine guns. 

Twelve hundred feet is generally considered as maximum ef¬ 
fective range. This does not mean that the bullets will do no damage 
beyond that range, but that bullet density will be too thin to justify 
firing. The air gunner has too small a supply of ammunition to 
waste any of it trying for chance hits beyond effective range. 

Example. 

For simplicity, the bullet pattern is here considered a square. 
A gunner fires 200 rounds at a target 1,200 feet away. At that 
distance his pattern covers an area 20 feet square; i.e., 400 square 
feet. Assuming that his aim is perfect and that the bullets are evenly 
distributed in the pattern, theoretically one bullet will land in every 
2 square feet. If the target at which he is aiming is 20 square feet 
(4 by 5 feet), 10 of his bullets will land on it. Thus he will score 
with 10 out of 200 shots for an average of 1 out of 20. 

If the gunner holds his fire until the range has closed to 600 feet, 
his bullet pattern will be slightly less than 10 feet square, or, 100 
square feet. If he now fires 200 rounds, 2 bullets will land in each 
square foot. Using the same target (20 square feet), he will get 
40 hits out of 200 shots for an average of 1 out of 5, or 4 times the 
number of hits he would have had at 1,200 feet. 

At 300 feet his pattern will be 25 square feet. Since his target 
is 20 square feet, 20/25 or 4/5 of his bullets will hit in it. He will 
score 160 hits for an average of 4 out of 5. (See fig. 63.) 

Thus we can see that by halving the range we quadruple the num¬ 
ber of hits. On the other hand, a gunner who shoots at 2,400 feet 
(twice the maximum range) can expect less than one-fourth the 
minimum number of hits that he would have scored at 1,200 feet. 

The figures above can readily be checked at a range of 100 feet. 


66 



Entire bullet pattern should be confined to the target whose area is 
1% feet square. Very few gunners will be able to fire this accurately, 
however. 

Firing 200 rounds at a target of 20 square feet, a gunner would 
get the following results if his aim were perfect: 


Range (feet) 

Hits 

Average 

2,400_ 

2 

1/100 

1,800_ 

4 

1/50 

1,200_ 

10 

1/20 

600_ 

40 

1/5 

300_ 

160 

4/5 


Bullet density depends in large measure upon bullet pattern for 
its determination. Therefore, in harmonizing a set of guns to pro¬ 
duce a desired fire effect, one must utilize the patterns of the indi¬ 
vidual guns. 



































Section 11. BORESIGHTING AND HARMONIZATION 


General. 

Adjusting the sights of a gun so it can be aimed accurately is 
called horesighting the gun. This involves bringing the axis of the 



sight and the axis of the bore into a known relationship, as for 
instance by making them intersect at a particular range. 

Convergent boresighting. 

Convergent boresighting is adjusting the sights of a gun so that 
the axis of sight and the axis of bore intersect at the usual firing range. 



From figure 64, it is clear that the gun will hit the point at which 
it is sighted at one range only (AC). It will shoot under the tar¬ 
get at range AB, and over it at range AD. 

This means that a gun can be fired accurately only at the range 
for which it was boresighted. 

For the BAM gun, .30 and .50 caliber, the range AC is usually 
chosen as 1,200 feet. 

To converge the line of sight and the axis of the bore at 1,200 
feet, we first fix the axis of the bore by setting the gun in a steady 
mount and sighting through the barrel at a point 1,200 feet away. 
We do this by removing the back plate and bolt and sighting di¬ 
rectly through the barrel, or by looking into a special barrel-sighting 
mirror which fits in the chamber. 


( 67 ) 














68 


Then, without disturbing the barrel, we lift our eye to the sight 
and, by moving the bead, align the peep and bead on the same point. 
The line of sight and the axis of the bore now converge at 1,200 feet. 
When the small area available or other circumstances make it incon¬ 
venient to use the full 1,200-foot range, we can obtain the same results 
by sighting at a specially prepared card, called a temq)late^ at a shorter 
distance, say 300 feet. 

Vertical olfset. 

The basic principle of this template can be understood by considering 
the similar triangles ABC and DEC in figure 66. 



If AC (the distance to the target) is 1,200 feet, and AD (the distance 
to the template) is 300 feet, and AB (the distance from the center of 
the bore to the center of the peep) is 5.0 inches, then, 

5" __ 1,200' 

DE 900' 

DE = ?>.7^" 

To converge the axis of sight and the axis of the bore at 1,200 feet, 
therefore, we align the axis of sight 3.75 inches above the axis of the 


A 


3 


Figure 67. —Template for horizontal offset. 

bore at 300 feet. (On any particular gun, of course, AB^ the height of 
the sight above the bore, must be carefully measured.) 

This correction allows, then, for the vertical offset of the sight. 

Horizontal offset. 

If the sights are offset horizontally (as most are) we must make a 
second correction. Precisely the same method is used as for vertical 
offset. 

If AC (the distance to the target) is 1,200 feet, and AD (the distance 
to the template) is 300 feet, and AB (the distance from the center of 
the bore to the center of the peep) is 1.5 inches, to the left, then, 

1.5 _ 1,200' 

DE 900' 



7>^=1.12" 






















69 


We should therefore align the axis of sight 1.12 inches to the left of 
the axis of the bore at 300 feet. (On any particular gun, of course, 
AB must be carefully measured. 

These corrections have allowed for vertical and horizontal offset of 
the sights. Our template will look as in figure 68. 

To the corrections for offset of sights we can add corrections for 
certain forces which affect the bullet after it leaves the gun. This 
can be illustrated by explaining correction for gravity drop. 


Am OF SIGHT 


I 3 . 75 " 

; . I 


Figure 68. —Sample template-—300 feet (gravity drop omitted). 

The gravity drop of a bullet traveling 1,200 feet is approximately 
4.5 feet. Instead of boresighting the axis of sight and the axis of the 
bore on the same point, we can boresight the axis of the bore 4.5 feet 
above the line of sight at 1,200 feet. This will cause the trajectory 
of the projectile to intersect the line of sight at 1,200 feet. 

This correction for gravity drop can be placed on our sighting 

template by raising the bore aligning mark of 4.5 feet or 13% 

inches. 



Our template (fig. 70) will now be modified. 

Both sample templates have assumed boresighting for 1,200 feet 
with an actual distance from peep to template of 300 feet. This is a 
good distance, but any convenient distance can be used. A distance 
of 50 feet is also goocl. The shorter the distance, however, the more 
serious the effect of even very slight inaccuracies in sighting and in 
placing the marks on the template. If it is at all possible, boresight¬ 
ing should always be done at full range (1,200 feet). 














70 


With convergent boresighting the trajectory, as shown in figure 
69, actually intersects the axis of sight in two places. With the sight 
axis 5 inches above the bore, most of the trajectory is above the axis of 
sight; it passes below at boresighting range. With an allowance of 
4.5 feet for drop at 1,200 feet, the trajectory follows the axis of sight 
very closely. Its maximum distance above is about 13 inches and it 
falls about lll^ feet below at 2,170 feet. These figures vary some¬ 
what, depending on the amount of vertical offset of the sights. 

Parallel boresighting. 

Under some circumstances, particularly for firing at very short 
ranges, and to simplify the use of the tracer method, squadron prac¬ 
tice calls not for convergent^ but for parallel^ boresighting. 


^ BORE mn 

k GRAVITY ALLOWAIVCE 


9.75" (13.S-S.75) 


SIGHT 

AXIS 


l.tZ" 



3.75" 

1 

^ BORE • /VO 

GRAVITY ALLOWANCE 


Figure 70.—Sample template—300 feet. (Offsets and drop.) 


It is easier to boresight a gun parallel, since the allowance for 
offset proportionate to the distance from the template need not be 
calculated. 

Horizontal and vertical offset are measured. Two crosses at cor¬ 
responding fuU-scale distances are made on a template. The tem¬ 
plate is set at any convenient distance, which need not be measured. 
The gun is then boresighted by directing the axis of sight toward the 
upper, and the axis of the bore toward the lower, of these crosses. 
The allowance for gravity drop may or may not be allowed for, de¬ 
pending on the circumstances involved. Some gunners prefer to 
shoot with tracers with the axis of the sight and the bore perfectly 
parallel. 

Harmonization. 

Harmonization is the aligning of a group of guns in relation to one 
another and to a gun sight in such a way that the greatest possible 
lethal effect is produced against the enemy at normal fighting ranges 




71 


under combat conditions. This involves (1) balancing the density 
required to drive off an enemy against the dispersion desirable to 
compensate for small errors in sighting, and (2) allowance for wander 
of aim. 

If the size of a bullet pattern is much less than the average sighting 



/o'm /o'-* 


Figure 71. 

error, the target may never be hit. If the pattern is very large it may 
not be dense enough to be effective, even if the target lies within it. 

Taking into account bullet pattern and the forces of exterior bal¬ 
listics, various lethal patterns can be set up. 

See figures 71, 72, 73. 

To effect the pattern of bullet dispersion shown in figure 74, a sight 
template would be made as in figure 75. 











72 



Figure 72. 




















73 



Two (2) .50 cal. 

Guns mounted in 
turret. 

Range - 1800 feet. 



Figure 73. 



35* 


CBNTEft OF FIRE 


Figure 74. 










































74 



, goeE»/ 

-- 

( 3 ) - 

1 

‘ < 

1 

1 

1 

' ^ center , 

1 

1 VoF FiRfe 1 


- 

1 

U'/z’eRAviTy 1 

1 

1 

J DROP , 

■^SIGHT , 

1 

1 

(2)- 

1 

- 1 

BORE ^3 



Figure 75. 

Four-gun turrets. 

If four .30-caliber guns in a turret are diverged toward the corners 
of a 7-foot 6-inch square at 1,200 feet, a circular pattern of fairly even 
distribution results, whose diameter subtends about 1°. Considering 
the areas included by 75 percent of the rounds fired, we can set up a 
table: 


Range (in feet)-- 

300 

600 

900 

1,200 

Diameter of pattern (feet).. ... _ 

5 

lOH 

15^ 

21 

Area of pattern (square feet). -- 

21 

85 

190 

340 

Bullets per square foot in 3 seconds_ 

11.5 

2.8 

1.3 

0.7 


Experiments with camera-guns show that a trained gunner is able 
to maintain a no-deflection aim accurate enough to keep an enemy 
within such a pattern about 75 percent of the time. Seventy-five per¬ 
cent of the figures for bullets per square foot at the various ranges 
become: 


Range (in feet)..... 

300 

600 

900 

1,200 


Density........ 

8.6 

2.0 

1.0 

0.5 



British firing trials against a captured enemy fighter show how the 
chance of driving off an enemy plane varies with bullet density per 
square foot: 


Density 

Chance 


Percent 

2 

93 

1 

72 

.5 

48 

.25 

30 





























75 


The chances of forcing an enemy to break off his attack by a 3-second 
Burst if the defender’s guns are harmonized in the way under discus¬ 
sion, can be computed for a no-deflection defense by combining the two 
sets of information. 


Range (in feet). .... 

300 

600 

900 

1,200 


Chance_ 

Percent 

100 

Percent 

93 

Percent 

72 

Percent 

48 



Two-gun turrets. 

In two-gun turrets, with divergent harmonization, the question 
arises whether to diverge vertically or horizontally. Vertical diver¬ 
gence is recommended when it is felt that most of the movement of 
enemy fighters at close range will be vertical. Consequently, vertical 
divergence would be more desirable for a tail gun and horizontal diver¬ 
gence for waist guns. Vertical divergence also can be utilized to allow 
for the variations of gravity drop with range. 


493667°—43-6 










Section XIL BALLISTICS 


Factors determining trajectory. 

The course a bullet takes after it leaves the muzzle is determined by 
(1) the characteristics of the bullet itself; (2) what happens to it before 
it leaves the muzzle; and (3) what happens to it after it leaves the 
muzzle. 

Some of these considerations are taken into account in boresighting 
and harmonization, some must be allowed for by the gunner, and some 
are so negligible in effect that they may be overlooked. The gunner 
needs to understand the effects of what forces he must correct for, and 
under what circumstances; and it is helpful for him to understand 
why the effects of certain other forces require no correction. 

Interior ballistics. 

What happens to the bullet before it leaves the muzzle is the concern 
of interior hallistics. It includes (1) the study of the combustion of 
the powder, the pressure developed, and the velocity of the projectile 
along the bore, and (2) the calculation of the dimensions of the powder 
chamber and of the powder which, for any particular design of gun 
and projectile, will give the required muzzle velocity, while not ex¬ 
ceeding the allowable interior pressure. 

Interior ballistics do not require the attention of the free gunner 
other than as indicated in the section on ammunition. 

Exterior ballistics. 

What happens to the bullet after it leaves the muzzle is the concern 
of exterior ballistics. 


FORCE OFmvm PLANE 


ROTATION 



PROPELLANT FORCE 


AIR RESISTANCE 


GRAVITY 

Figure 76. —Forces acting on bullet. 


Five forces act on the bullet in flight: 

1. Propellant force, which is the result of the burning of the powder 
in the case. The amount and type of powder determines the speed 
of the bullet. The faster a bullet reaches the target the less time 
the other four forces have to act on it. 


( 76 ) 









77 


2. Gravity, which begins to pull the bullet straight clown as soon 
as it leaves the muzzle. The drop due to gravity varies directly with 
the range—the farther the bullet goes, the longer the time it has to 
fall—and inversely with the speed—the faster it goes, the shorter the 
time it has to fall. The distance it falls in a given time can be com¬ 
puted from the formula: 

Distance=i /2 GT^, where G is 32 and T is time of fall in seconds.^ 

3. Air resistance^ which causes the bullet to slow down. It increases 
with speed and, because air is less dense higher up, decreases with 
altitude. By studying the speeds of a projectile at various ranges 
(p. 56), it can readily be seen that the projectile slows down at a very 
high rate at first, and as the velocity decreases, the effect of air 
resistance decreases and the deceleration is less rapid. 

4. Force due to motion of the airplane^ which causes the bullet to 
move after it leaves the muzzle in the same direction and with the 
same speed as it was being carried by the airplane. 

5. Rotation of the hullet^ which is caused by the rifling (spiral 
scoring) of the barrel. (American rifling is clockwise viewed from 
the rear.) In the .30-caliber BAM gun, there is 1 rotation for every 
10 inches of barrel travel. In the .50 caliber 1 rotation for every 15 
inches of barrel travel is completed. 

Precession. 

Rotating bodies, such as tops, baseballs, and bullets, display certain 
phenomena called gyroscopic. These phenomena are best understood 
by considering a model gyroscope. 



Figure 77.—Gyroscope. 


1 Though this formula applies to a body falling in a vacuum, and therefore takes no 
account of air resistance on the way down, it is sufficiently accurate for practical computa¬ 
tion. In a vacuum a body falls 16 feet the first second and accelerates at the rate of 62 
feet per second for each successive second. 

Illustration: How far does a bullet fall in 0.5 second? Using the formula—Distance = 
0.5 X 32 X 0 . 52 , we find the distance to be 4 feet. 






















78 


The wheel is spinning on the axle AA', which turns in the 
ring T. The ring “7” is free to rotate on the axle BB', which can 
turn in the ring U. The ring is free to rotate on the axle 00 , 
which can turn in the frame V. Gyroscopic phenomena will occur 
only while the wheel “/S'” is spinning. This model may be described as 
having “3° of freedom,” meaning that motion is possible in three direc¬ 
tions, each at right angles to the other two. 

If downward pressure is applied to ring T at point A, instead of 
rotation of T about axle BB' as might be expected, we get rotation of 
U about axle 00' in a counterclockwise direction, viewed from above. 
Further, any turning applied to a part of the model will result in 
rotation at 90° to the direction in which the pressure is applied. This 
motion is called precession. 

The rule may be stated: A force tending to turn the wheel about 
some axis XX'., will produce a turn about such another axis FT', and 
in such a direction as Avill tend (1) to make the axle of the spinning 
wheel coincide with the axis XX' and (2) to make the spin of the 
wheel, when this position is reached, coincide in direction with the 
force applied around the axis XX'. 

The rotation of the bullet gives it gyroscopic properties. The point 
of application of the resultant air resistance is in front of the center 
of gravity. If the bullet were not spinning, the effect of this pres¬ 
sure would be to cause rotation about an axis through the center of 
gravity and perpendicular to the plane of fire, i. e., perpendicular 
to this page (fig. 78). This would cause the bullet to tumble counter¬ 
clockwise. Actually, the air pressure causes precession. With clock¬ 
wise rotation, the point of the bullet turns to the right. Then the 
left side of the point is subjected to pressure and the point tips down. 
The tipping down keeps the, axis of the bullet essentially tangent to 
the trajectory during its entire flight and causes the hullet to follow 
the trajectory head on. 

The gyroscopic motion of the projectile is responsible for the sta¬ 
bility of the bullet during flight. Precession of the point is the result 
of air pressure and causes the bullet first, to drift, and second, to 
change the inclination of the longitudinal axis in a vertical plane so 
that this axis remains essentially tangent to the trajectory. 

It is normally considered that the speed of rotation of the projectile 
necessary to maintain satisfactory gyroscopic action is approximately 
1,600 revolutions per second. With the initial muzzle velocity at 
2,700 feet per second, the projectile will spin approximately 3,200 
revolutions per second. 

These five forces interact to cause a deflection of the bullet from the 
projected axis of the bore. This deflection, called trail, is made up 
of five components: (1) force due to motion of firing aircraft; 
(2) drop; (3) jump; (4) yaw; (5) drift. 


79 


The propellant force and the force due to the motion of the air¬ 
plane are the chief factors in determining the course of the bullet. If 
from a point representing the muzzle of the gun lines are drawn 
proportionate in magnitude to the speed of these two forces and at the 
same angle as that which the direction of the gun bears to the direc¬ 
tion of the plane, and if a line is drawn parallel to each of them in 



such a way as to form a parallelogram, the diagonal of the parallelo¬ 
gram will represent, in magnitude and direction, the resultant force 
on the bullet. For instance, if a plane were going 100 knots (169 feet 
per second), these forces acting on a .30-cal. bullet fired at 90° would 
be represented by lines 169 and 2,700 units long, drawn at 90°. The 
diagonal will represent the resultant force. Gyroscopic action of 
the projectile causes it to head into its resultant trajectory. It is 



Figure 79.—Force due to motion of firing aircraft. 


important at this point to note that the resultant trajectory is formed 
by the initial muzzle velocity (2,700 feet per second) and the speed of 
the firing aircraft. Leaving out of consideration other ballistic 
forces, the bullet will maintain this resultant trajectory throughout 
its flight. 

Air resistance acts by itself in slowing down the bullet head on, 
and in combination with other forces in causing yoAX)^ drifts and 
jump. 

Gravity by itself exerts a downward force called drop, and com¬ 
bines wdth air resistance and with the force of rotation to cause a drift 
to the right. 







80 

Kotation acts in combination with air resistance and gravity to 
cause drifts and yam. 

1. Force due to firing aircraft.—Lag is the correction for the force 
due to the firing aircraft. It always occurs along the line of motion 
of the gunner’s aircraft, in the same direction as the aircraft’s motion. 
It is automatically allowed for in apparent speed sighting. 

2. Drop.^The distance gravity pulls the bullet downward is com¬ 
puted from the formula : Distance = 146^7'^, where G is 32 and T is 
the time in seconds. Drop is corrected for in boresighting and har¬ 
monizing. Although this correction is accurate only for one range 



and for horizontal fire, the errors at other ranges and angles are 
slight. 

For the BAM .30-cal. gun, the maximum error caused by gravity 
drop, when firing horizontally up to a range of 1,500 feet, for the 
center of the cone of fire (fig. 80), is about 3 feet. 

It should be noted that when a gun is boresighted to allow for 4.5 
feet gravity drop when firing on the horizontal, errors will occur when 
the barrel is elevated or depressed from the horizon. The maximum 
error, 4.5 feet, results when firing vertically up or down. (See fig. 81.) 

Horizontal—Trajectory and sight axis converge. 

30°—Trajectory 2.25 feet above sight axis. 

45°—Trajectory 3.15 feet above sight axis. 

Vertical—Trajectory 4.50 feet above sight axis. 

3. Jump .—^Jump is the resultant of bullet rotation and the wind 
resistance due to the motion of the firing airplane. It causes the pro¬ 
jectile to climb when shot to port, to veer left when shot dowmward, to 
fall when shot to starboard, and to veer right when shot upward. 

Jump is caused by three forces: 

1. The roll of the bullet against the “wall” of air pressure. 

2. The aerodynamic force of the air flow past the rotating bullet. 

3. The gyroscopic action of the projectile. 










81 


UJ 

or: 

o 

cs 


o 




/ A 

/ 

/ 

/ 

/ 

^ aatfoaavis ^ ' 

1533 

/ 

/ 

/ 

/ 

V ^ 

\ 

i 


figure 82.—Direction of .jump in relation to direction of firing. 




Figure 83,—Jump analogy of hoop. 


























82 


The first two forces are very slight and have very little effect on the 
trajectory of the projectile: (1) the roll of the bullet against the 
“wall” of air pressure, can be explained by the analogy of a child’s 
hoop. 

When a child’s hoop is given a rapid twirl and dropped upright on 
the ground, it rolls off in the direction in which the top of it is turning. 



(Bullet is moving straight into page.) 

A bullet fired from an airplane behaves similarly. It is spinning 
when it leaves the muzzle. Because of the speed of the airplane, the 
resistance of the air against the side of the bullet, unless it is shot 
straight forward or aft, presents a relatively solid surface. On this 
surface the bullet rolls. 

(2) The aero-dynamic force of the air flow past the rotating 
bullet acts in the opposite direction from the jump but has very little 
effect on the trajectory of the projectile. 



In figure 85, the rotating body is turning clockwise and the flow of 
air is from left to right. The flow of air above the rotating body 
is in the same direction as the circular flow of the body. Hence the 
pressure will be slightly reduced due to the speeding up of the air 
flow. The flow of air on the under side of the rotating body is in the 
opposite direction from the rotation of the rotating body; hence the 
pressures will be increased due to the slowing down of the air flow. 
With a lower pressure on the upper side and a higher pressure on the 































83 

under side, the rotating body will tend to rise toward the side with 
the lesser air pressure. 

(3) The major force causing jump is caused by the gyroscopic ac¬ 
tion of the projectile. The instant the projectile leaves the barrel it 
is struck by a blast of air (slip stream) which causes it to process. 
During the first quarter cycle of its precession, it heads at right 
angles to the slip stream, as indicated in figure 86. 

Due to the speed by which the projectile turns into the resultant air 
flow this force of jump ends in the first quarter cycle of the projectile, 
causing the projectile to assume a new heading in the direction of the 
jump. 

The gunner should realize that this action takes place almost instan¬ 
taneously as the projectile leaves the barrel and even though this force 
is over in about the same length of time it starts the projectile off on a 
new heading which it maintains throughout its trajectory. 

Because jump causes a change in the heading of the projectile the 
effect is uniform throughout the trajectory. That is, with a given 

I PS r R E A M 

Figure 86. 

speed of rotation and a given speed of the moving platform, if the 
bullet is deflected downward 3 feet at 1,000, it will be deflected 4^2 feet 
at 1,500 or 6 feet at 2,000. 

The approximate jump in mils for a BAIVI gun is computed by 
multiplying the true air speed in knots by a constant; for the .30 by 
0.0152, for the .50 by 0.0098. True air speed is always a little greater 
than the air speed indicated by the airplane’s instruments. It varies 
from 1.045 times the indicated air speed at 3,000 feet to 1.56 times it at 
27,000 feet. 

The allowance to be made for jump at 1,200-foot range by a .50- 
caliber gunner flying at 3,000 feet at an indicated air speed of 300 knots 
would be found thus: 

300 X 1.045 X 0.0098=3.07 mils 

3.07 (feet per thousand feet) X 1.2 (thousands of feet) =3.68 feet (jump 

at 1,200 feet) 

A correction for jump can be boresighted into a gun if the gunner’s 
field of fire is confined to a small area. Thus in a port gun allowance 
could be made for the upward jump. This correction will take care of 
any angle of elevation, but the gunner will have to compensate for the 





84 


boresighting corrections as he changes the azimuth of his gun from a 
straight beam shot. 

4. Yaw .—Yaw is an angular deviation of the longitudinal axis of a 
bullet from its trajectory caused by the irregularities of the bullet and 
of the barrel and by the blast of relative wind on the forward side of 



Figure 87.^—Yaw. 


the bullet as it leaves the muzzle. The bullet wobbles or spirals around 
its trajectory. 

Yaw dampens out very quickly. Even with the most radical yaw 
the projectile assumes a stable course in the first 0.8 second of flight. 
The effect of yaw is simply to slow down the bullet. (The resistance 
of a bullet with yaw of 13° is about double that of one without yaw.) 



Figure 88.—Drift. 


No correction is made for yaw in sighting or in boresighting because 
the average yaw is considered in the calculation of the speed of the 
projectile over various ranges. 

5. Drift .—Drift is analogous to jump, except that the bullet rolls, 
not across the relative wind set up alongside it by the forward motion 
of the plane, but across the air beneath it which resists the bullet’s 
downward fall. 

Drift is always to the right and increases with range, because the air 
resistance increases as the bullet accelerates downward. The faster 
the bullet falls, the greater this pressure becomes and consequently the 
greater the drift. Drift is omitted from all aerial gunnery calcula¬ 
tions because it is practically negligible (only about 1 foot at 3,000 
feet). 








85 


Summary of ballistic forces 


Force 

Cause 

Magnitude 

Direction 

Correction 

1. Yaw . 

2. Drift- . 

3. Drop _ 

Imperfections in barrel and 
in bullet. 

Gravity vs. rotation of 
bullet. 

Gravity.- .... . . 

Slows bullet. 

1 foot in 3,000_ 

U32T2 . _ 

Along trajectory... 

To the right_ 

Down . _ 

None. 

None. 

In boresighting. 
Partly in boresight¬ 
ing; partly allow¬ 
ance when sighting. 
In sighting. 

4. Jump _ 

5. Lag--.. 

Rotation vs. air resistance. 

fPropellant force_ 

\Motion of plane_ 

See p. 122_ 

1 Varies... _ 

See fig. 82. 

Direction of flight. 


Note.—Y aw is also caused by lateral force imparted to the projectile by speed of firing plane. 

Apparent wind deflection. 

The experienced gunner also makes allowance for apparent wind 
deflection, caused by the bullet’s loss of speed due to air resistance. 



Because the target plane ordinarily maintains its speed undimin¬ 
ished, and because the bullet does not, the trajectory of the bullet 
appears to the gunner to curve in a direction opposite to the fliglit 
of the target plane. 

This is not a real deflection, like jump or drift or drop. There is 
no bullet trail. What happens can best be shown by a diagram 
(fig. 90). 

At the left the firing plane is represented at successive tenths 
of a second. [A, C _ G.) The line A, 6, c- g repre¬ 

sents the trajectory of a bullet fired from position A at the target 
plane flying parallel at the same speed. Ibis trajectory is drawn 
exclusive of jump and drift. The position of the bullet is repre¬ 
sented at successive tenths of a second (&, <?,- g)- 


































The bullet cactually travels straight. But to the gunner it seems 
to fall back, as is shown by the increasing distance of points c, d, 

/, behind the successive intersections of the trajectory, and the 


FIRING PLANE 
G 




AXIS OF BORE TARGEF 


APPARENT 
WIND DEFLECTION 


IT % 


"bjo' 


'/o 








Mo 






- ^ -T- 4 










5^0 

-T- 1’^'^ 

^r=i- 

'Xc 


/ 3 






10 & 


C.V 






M'o 


■ I ' J 


trajectory exclusive 
OF JUMP, PRIFT, OR 
y/ijv. 


/O 


10 


T 


Figure 00.—Explanation of apparent wind deflection. 




positions the bullet should have reached if it had maintained its 
initial velocity. 

Apparent wind deflection increases with range (bullet has more 
time to slow down) and the speed of the target. 

Trail tables (BAM .SO gun) 

These figures are based on experiments with targets towed at 3,000 
feet in the same direction and speed as the firing plane, muzzle velocity 
2,700 feet per second. Trail is given in feet. 


[Indicated airspeed 100 m. p. h,] 



600-foot range 

1,800-foot range 

Vertical 

Lateral 

Vertical 

Lateral 

Firing on tbe horizon- 

[Bow_ 

Stern. _ 

Port beam... 
Starboard 
beam.. ... 

-0. 96 
-0. 96 
-0.12 

-1. 74 

0 

0 

L 2.28 

R 2. 28 

-10. 44 
-9. 72 
-7. 74 

-12.6 

0 

0 

L 23.04 

R 23.04 


.\ft 

Lateral 

Aft 

Lateral 

Firing straight up... 

Firing straight down.. 

-2. 28 

R 0. 84 

-23. 22 

R 2. 52 

-2. 28 

L 0.84 

-22. 68 

L 2.44 















































87 


Indicated airspeed 250 m. p. h. 



600-foot range 

1,800-foot range 

Vertical 

Lateral 

Vertical 

Lateral 

Firing on(®°7n .. 

-1Starboard beain._. 

-0.96 
-0.9 
-1.08 
-3.0 

0 

0 

L 2.38 

R 2. 38 

-11.16 
-9. 36 
-4. 68 
-16. 92 

0 

0 

L 69. 84 

R 69. 84 


Aft 

Right and 
left 

Aft 

Right and 
left 

Firing straight up,_. 

Firing straight down. 

7.38 
-7.38 

R 2.04 

L 2.04 

70. 38 
-68. 94 

R 6.12 

L 6.12 


t 


Trail tables BAM .50 (jmi 

This table gives computed vertical and horizontal trail for caliber 
.50, Ml ball cartridges fired with a muzzle velocity of 2,700 feet per 
second at an altitude of 3,000 feet from an aircraft in horizontal flight 
at 100 miles per hour. Range and trail are given in feet. 



600 

900 

1,500 

Azimuth (degrees) 

Vertical 

Lateral 

Vertical 

Lateral 

Vertical 

Lateral 

(0 _ 

45_ 

90_ 

Horizontal firing, 135_ .. 

180_ 

225_ 

270_ 

.315_ 

-0.9 
-1.2 
-1. 2 
-1. 2 
-0.9 
-0.3 
-0.3 
-0.6 

0 

R 0.9 

R 1.5 

R 0.9 

0 

L 0.9 

L 1. 5 

L 0. 9 

-2.1 
-2.4 
-2.7 
-2.4 
-1.8 
-1. 2 
-1.2 
-1.5 

0 

R 2.4 

R 3. 3 

R 2.4 

0 

L 2. 4 

L 3.3 

L 2.4 

-5.7 
-6.6 
-6.9 
-6.6 
-5.7 
-4.8 
-4. 5 
-4.8 

0 

R 6.9 

R 9.6 

R 6.6 

0 

L 6.6 

L 9. 6 

L 6.9 


Lateral 

Fore 
and aft 

Lateral 

Fore 
and aft 

Lateral 

Fore 
and aft 

Straight up...- 

R .6 

1.5 

R .9 

3.3 

R 1. 2 

9.9 

Straight down.. 

L .6 

-1.5 

L .9 

-3.3 

L 1. 2 

-9.6 
















































Section XIII. OPTICAL SIGHTS 


Purpose. 

The prime function of any aerial gun sight is to define a sighting 
line at some point along which the enemy airplane and the fired 
projectile will collide. This sighting line may be defined by either 
mechanical or optical means. 

Mechanical sights. 

The ordinary aerial gun sight is composed of a ring peep and 
bead. This is merely an elaboration of the familiar rifle head and 
peep sight with a ring encircling the peep to aid in deflection shooting. 

In order to fire with a lead which will enable him to hit, the 
gunner uses his ring sight to estimate the range and apparent speed 


Figure 91.-—Multiple ring sight with angle lines. 



and direction of the target. Some sights have been made with mul¬ 
tiple rings and angle lines^ but they are more confusing than helpful 
to the gunner in solving these problems. 

Optical sights. 

To remove the difficulties encountered in the use of mechanical 
.sights, optical sights have been devised. At times both types are 
mounted on a gun, the more sturdy mechanical sight being ready for 
use in case the more fragile optical sight goes out of commission. 
Also with the optical sights may be found accessory devices, the 
most common being a range finder; e. g., Sperry self-computing 
range finder. 

Reflector sights. 

(1) General .—reflector sight serves to eliminate the errors of 
mechanical ring and post sights which are introduced when the gun¬ 
ner fails to: 


( 88 ) 







89 


(a) Keep his eye at the correct distance from the ring to maintain a con¬ 

stant sight base. 

(b) Align the bead exactly in the center of the peep. 

(c) Focus his eye simultaneously both on his target off in the distance 

and on his ring only 20 inches away. 

(d) See his sights distinctly at night. 

The optical system of a reflector sight consists of a light source, 
a reticle, a projection lens system, and a reflecting plate. The bulb 
contains two incandescent filaments, and the intensity of its illumi¬ 
nation may be adjusted by a rheostat to fit existent light conditions. 
The reticle is an otherwise opaque plate on which lines and figures, 
usually in the form of a ring and bead, have been etched. When 
the bulb’s light rays hit the reticle, the pattern is projected, through 
a lens system onto a transparent optical flat, the reflector plate. 



Figure 92.—Diagram of reflector sight. 


through which the target is also visible. The reflector plate, made of 
special triplex glass, is interposed on the line of sight at 45°. (See 
fig. 92.) 

By this arrangement the lens system picks up the light rays 
diverging from points on the reticle and projects them as parallel 
bundles of rays. These rays are interrupted by the inclined reflector 
plate, where the image is viewed by the eye. The image then ap¬ 
pears to be always in the same direction from the observer and at 
an infinite distance. 

As a result, the sight rings seem to be large rings seen at a great 
distance superimposed on the target plane. As long as the rings can 
be seen, correct alignment of eye, bead, and peep is assured. If the 
gunner moves his head from side to side or back and forth, the size 
and position of the target plane do not change relative to the size and 
position of the ring. For the position and size of the target to change 


























90 


in the ring there must be relative movement betv/een the firing and 
target planes. 

(2) Advantages .—In the reflector sight, the sight base is kept con¬ 
stant, the bead is always lined up in the peep, and the gunner has no 
trouble focusing his eye simultaneously on his sight and on his dis¬ 
tant target. He can adjust the brightness of the ring for varying 
light conditions, and a sun filter, which may be raised or lowered, 
prevents him from being blinded when he points the gun into the sun. 
His field of view is unrestricted except for the sight’s supports, and 
the loss of light passing through the reflector screen is less than 10 
percent. 

(3) Disadvantages .—Reflector sights of all types are costly, and 
their production requires valuable time and labor. In combat they 
present a large vulnerable surface often extending to the airplane’s 
electrical system. These sights require more over-all maintenance than 
do mechanical ones. Bulbs burn out and lenses tend to fog up, espe¬ 
cially at high altitudes. 

The following are some of the reticles found on reflector sights. 

(O) 

35 '70 mil rings 

Figure 93 




_ 300 








./tfOO 


Christmas tree 

-Reticles used in reflector sights. 


The Christmas Tree sight is of no aid to the air gunner in estimating 
range or in deflection shooting, yet it has not been entirely discarded. 
The horizontal bars are merely set in boresighting so that when a 
gunner puts a target plane on that bar corresponding to the plane’s 
range, he is making the proper allowance for gravity drop. This sight 
has no ring for deflection allowance. 

Namy Mark 7 is similar to the Army N2A, N2AN, N3A, and NBA 
sights. It is the largest of the reflector sights, too bulky for most 
flexible guns; most commonly used in turret and fixed-gun installa¬ 
tions. 

Mark ^.—Medium sized; contains leveling bubble to enable fixed 
gunner to compensate for skids of own plane. 

Mark Smallest, least exposed, most serviceable of reflector 
sights; housed in fragile bakelite case; difficult to clamp on gun, most 
easily used with hand held flexible guns. ’ 





91 




Figure 95.—Mark 8 sight. 



i'. 

i; 


498fifi7°—43 


■7 


































92 


Sperry automatic computing sight.— See Sperry turret. 

Pre-fiiglit inspection .—Actual internal inspection should be made 
only by qualified personnel thoroughly familiar with the operation of 
reflector sights. But a gunner should turn on his sight before each 
hop, see that plates are clean, electrical contact is good, bulb is bright, 
replacement bulbs are available, supports are firm, and that there is 
no sign of fogging in the lenses. He should look through his sight, 
move his head from side to side, making sure that the relation of the 
reticle image to any distant object does not change; i. e., that there 
is no parallax. 

Telescopic sights. 

Some free aerial machine guns today are equipped with telescopic 
sights. In most cases they have one eyepiece with a rubber buffer, 
and a fixed focus. They offer practically the same advantages over 
mechanical sights as do reflector sights. 

1. Advantages .—(«) The gunner focuses his eye on only one dis¬ 
tance instead of trying to accommodate for more than one, as with the 
ring and bead. The sight markings, in the form of rings or intersect¬ 
ing lines, and the image of the target are both seen in the same plane. 

( h ) The gunner is not forced to keep his head in an absolutely fixed 
position, since slight motion of the eye either along or across the sight¬ 
ing axis will not affect the accuracy of his aiming. 

2. Disadvantages. —(a) The gunner’s field of vision is cut down. 

(J) As contrasted with the reflector sight, the telescopic sight 

allows the gunner very little head movement. It is comparatively 
easy for him, especially in swinging the gun fast, to move his eye out 
of line, and the danger of black-out is acute. 

(c) Although not dependent on electrical current, the telescopic 
sight is an intricate, delicately balanced mechanism, which is hard to 
produce, offers a large vulnerable surface, and is easily damaged. 

id) In spite of vent holes, telescopic sights often fog in flight. 











Section XIV. TURRETS 


General. 

A turret is a small, cylindrical, or spherical structure which re¬ 
volves and within which heavy guns are mounted. At the present 
time there are four main types in operation; (1) the upper turi'et^ 
which is located in the upper part of the plane for the protection 
of the plane’s itpper hemisphere; (2) the lower twrret^ which is 
located in the lower part of the plane to protect the lower hemi¬ 
sphere; (3) the rear or tail turret^ which protects the plane’s after- 
quarter; (4) the how twrret^ which protects the plane’s forward 
area. 

As aircraft adopted heavier armor plating, it was found necessary 
to use higher-caliber guns to shoot them down. These heavier 
weapons required more rigid mountings. In fighter planes and 
other highly maneuverable small craft they were built in as fixed 
guns. But, since fixed guns cannot afford large bombers the field 
of fire they require for protection, the engineers had to find some 
means of mounting the heavier weapons which combined the firm¬ 
ness of the fixed gun and the freedom of movement of the flexible 
gun. The turret was the answer. 

Though turrets add to a plane’s weight, they reduce its comple¬ 
ment without detracting from its fire power. With hand-operated 
guns, one operator is required for each weapon, while in turrets 
one gunner can control the fire of several guns. From a turret, 
the gunner’s fire should also be more accurate since he does not 
face the problem of keeping a hand-held gun steady in the slip 
stream. 

All effective turrets are power-operated. The power, which serves 
to drive the turrets in azimuth and the guns in elevation, may be 
either electrical, hydraulic, or a combination of the two. However, 
during repairs or in emergencies the turrets may be controlled 
by hand. 

A variety of sights is used in the turrets, but all are of the optical 
type. The turret sight, instead of being mounted directly on the 
guns as on hand-held guns, is set on a sighting cradle between the 
guns. The entire sight rotates with the turret in azimuth and with 
the guns in elevation. 

Most turrets, and all those discussed below mount two .50-caliber 
Browning machine guns and carry approximately 400 rounds of 

( 93 ) 


94 



ammunition for each gun. The guns have recoil absorbers which re¬ 
duce bullet dispersion and lessen vibration within the turret. In 
some turrets the guns may be fired either individually or together, 
as the gunner desires. In others they must both be fired together. 

The amount of movement of a turret and also the field of fire of 
its guns depend upon the type of plane in which the turret is mounted 
and upon its position in the plane. No matter how freely a turret 


Figure 98.—Martin turret—upper. 

and its guns may swing in azimuth and elevation, fire-interrupter 
cams and switches prevent a gunner from firing into his own plane. 

MARTIN UPPER TURRET 

General. 

1. Use .—Mounted in the top of the fuselages of B-24's, B-26*s, and 
PBM's to protect their upper hemispheres. 

2. Guns .— Tavo .50-caliber M-2 machine guns. 

3. Power. —Electric. 

4. Movenfient .—Turret moves 360° in azimuth; guns from horizontal 
to 85° in elevation. 

5. Hand control .—“Potentiometer control unit” has two spade grips 
Avhich move turret in azimuth and guns in elevation. A simple twist 




95 


of the wrist causes the guns and turret to move in the direction of the 
twist. 

6. Speed. —The speed of rotation or elevation is controlled by the 
amount of angular displacement of the hand control. Small angular 
motions produce slow speeds; large angular movements result in fast 
speeds. Depressing high-speed switch on right-hand control doubles 
speed. 

7. Ammunition. —Located in front of the gunner’s knees are two 
ammunition cans, each containing a maximum of 400 rounds. Each 
gun is fed by a booster motor. 

8. Charging. —Guns are charged manually by means of a folding 
charging handle. They must be set at an elevation of 45° before being 
charged. 

9. Sight. —N2A or N3A. (See section on Optical Sights.) 

Operation. 

1. Close master gun switch, below right elbow. 

2. Close azimuth and elevation motor switches, on front of junction 
box beneath seat. 

3. Adjust sight illumination by means of sight rheostat, on left side 
of sight. 

4. Depress dead man switch on left-hand control grip to enable 
turret to move. 

5. If extreme speed is desired, depress high-speed switch above 
right-hand grip. Do not keep on for more than 30 seconds at a time. 

6. Charge guns. 

7. Track target in sight. 

S. Fire guns singly or together by closing trigger switches on spade 
grips. 

9. When through firing, open master gun switch and azimuth and 
elevation motor switches. 

Special features. 

In general the Martin turret has proved the most successful of the 
turrets now in use. Lightest in weight, it is the least burden for a 
plane to carry. It stands up well under combat conditions and requires 
less over-all maintenance. The twm-handed-control unit makes it 
highly maneuverable and enables the gunner to catch sight of and 
track his target easily and steadily. 

The Martin turret is operated electrically, working off the electrical 
system of the aircraft. Unlike the other turrets it has no hydraulic 
units to freeze at high altitudes. Unlike the guns in Bendix and 
Sperry turrets which have hydraulic gun chargers, the Martin turret 
guns are hand charged. They are fired by means of a solenoid. 


96 


SPERRY UPPER LOCAL TURRET 

General. 

1. Use. —Mounted in top of B-17E just to the rear of pilot’s com¬ 
partment for protection of plane’s upper hemisphere. 

2. Guns. —Two .50-caliber machine guns. 

3. Power. —The source of power is electrical, but the transmitted 
power is hydraulic. 

4. Movement.—l-w azimuth the turret moves 360°; the guns are 
limited in their movement to 85° in elevation. 

5. Hand control. —Gunner moves two control handles like bicycle 
handle bars to control the movement of guns and turret. 

6. Speed. —The guns move in elevation and the turret in azimuth 
at a rate of from i/4° to 11.4° per second. The maximum speeds are 
45° per second in azimuth and 30° per second in elevation. These 
speeds may be obtained independently or together, as is desired. How¬ 
ever, at speeds above 11.4° the sight fails to compute properly. 

7. Ammunition. —400 rounds per gun. Packed in magazines lo¬ 
cated just below the guns, the ammunition is fed from the inside 
through guides and rollers. 

8. Charging. —Guns are charged manually. Gunner pulls and re¬ 
leases charging handles located overhead. The turret also has an 
arm with a forked end which can be used to lock the charging handles 
down, thereby holding the gun bolts to the rear in a safe position. 

9. Sight. —Sperry Self Computing K-3. (See “Special Features.”) 
Operation. 

A. Before Take-Off .— 

1. Clean plexiglass panels. 

2. Check to see that there is oil in breathers, that power clutches 
are engaged and hand cranks disengaged. 

3. Load ammunition cans and guns. 

4. Turn on main switch, on switch box. 

5. Grasp grips thereby closing “dead man” switches which 
prevent turret from moving and guns from being fired when 
gunner is not at controls. 

6. Turn on sight switch. 

7. Allow hydraulic units and sight to warm up. 

8. Check to see turret responds to azimuth and elevation con¬ 
trols. 

9. Turn range knob to see that sight is connected and reticles 
operate. Then adjust reticle light to desired brilliance. 

10. Charge guns twice and leave in safety position. 


97 


B. After Take-Off .— 

1. Release charging cables from safety positions. 

2. Set target dimensions dial. 

3. Turn on fire-selector switches on switch box. 

4. Turn controls until reticles are superimposed on target. 
Then continue tracking it. 

5. Adjust range knob until reticle frames target. 

6. To fire, close trigger switches just in front of each grip. 

7. When through firing, charge guns at least twice to clear out 
all live shells. 

8. Leave guns in safety position. 

9. Set guns at 180° azimuth, 0° elevation. 

Special features. 

Despite their dependence on hydraulic power units which tend to 
freeze up at high altitude, the Sperry turrets have proved highly satis¬ 
factory. 

In the upper local turret the gunner stands on a platform which is 
mounted on beams which enable him to turn with the turret so that 
his line of vision will always be in the same general direction as the 
guns. 

The electrical switch box contains two gun-selector switches which 
allow the guns to fire separately or together, as the gunner desires. 
There is a firing switch on each grip for the gunner’s left- and right- 
hand forefingers. If both gun-selector switches are on, or if one or 
both of the dead-man switches are on, either of the firing switches will 
fire both guns. Thus, with one hand, the gunner can fire both his guns 
and operate the turret. 

Sperry self-computing sight. 

The Sperry .50-caliber automatic computing sights, although not 
limited in their application to any particular airplane or assembly, 
are specifically designed for use with Sperry turrets and are their most 
notable feature. 


Type : used in— 

K-3_Sperry upper local turret. 

K-4_Sperry ball turret. 

K-5_Sperry lower remote turret. 


The sight is mounted either on a cradle which is rigidly attached 
to the guns or on a sighting station which always maintains its align¬ 
ment with the remotely located guns. In either case the sight moves 
in azimuth and elevation with the guns. 

Operation .—The sight consists of an optical system, a range finder, 
and an automatic computing mechanism, all combined in a single unit. 





98 


The optical system is used to locate the target in space; the computing 
mechanism solves the problems of target deflection and ballistic de¬ 
flection, and automatically transmits these solutions to the optical 
system. The line of sight is thus offset from the gun bore in propor¬ 
tion to the calculated deflections. When the gunner moves the sights 
(and therefore the guns) so that the line of sight is realigned on the 
target, the necessary allowances are automatically made, and the pro¬ 
jectile will be properly directed to hit the moving target. 

Range is solved through the medium of the optical system. A hori¬ 
zontal bar of light passes through the reticle at all times and remains 
in one position. Vertical light bars may be set in different positions 
perpendicular to the horizontal bar of light. By turning the range 
knob the gunner adjusts these vertical bars to frame his target. 

Through his knowledge of the enemy aircraft the gunner knows the 
dimensions of his target. He sets in the target dimension by means 
of the target-dimension knob and dial. This dimension, once set in, 
can be taken as constant for all subsequent target positions and ranges. 
So that he may properly frame his target, the gunner moves the sight 
with the target to keep it in his line of sight. While he tracks the 
target he adjusts the range knob to position the vertical light bars so 
that the wing tips of the target will be properly framed between them. 
Once his target is framed, the gunner is ready to fire. By continually 
adjusting the range knob, he solves the range for the entire course of 
the target. 

Errors .—Sperry sight is nongyroscopic and therefore can be 
used with absolute accuracy only when it is level or when the plane in 
which it is mounted is in level flight. When a plane banks, the Sperry 
sight will compute improper deflection allowances. The errors may 
be large, varying with tlie sharpness of the plane’s bank. 

SPERRY LOWER BALL TURRET 

General. 

1. Mounted in the belly of B-17’s in same position as the lower 
remote turret it is designed to replace. 

2. Guns .—Two .50-caliber M-2 machine guns. 

3. Potcer.—-Same source as upper local. An electric motor drives 
two hydraulic units which in turn drive the turret in azimuth and the 
guns in elevation. 

4. Movement .—ball turns a complete circle. It can move in 
azimuth and 90° in elevation. 

5. Hand control.—K pair of hand grips extend from the azimuth 
and elevation hydraulic units. Tlie gunner grasps the grips and turns 
them in azimuth or elevation to control turret motion in these planes. 


99 


The two handgrips always move together. When released by the 
gunner they return to their center. 

6. Charging. —Same as upper local. 

7. Sight. —Sperry automatic computing sight K-4. 

Operation. 

See upper local description for general operational steps. This tur¬ 
ret must be entered from inside plane by carrying out the following 
procedure: 

1. Engage outside hand crank, located outside of ball. 

2. Unlock hand crank shaft. 

3. Turn turret until guns are at 90°. 

4. Open turret door, engage inside power clutch, and stow outside 
hand crank. 

5. Disengage outside hand crank clutch and lock shaft. 

6. Enter turret. 

7. Check all clutches, close door, and turn on power. 



figure 99. 


Special features. 

The turret has two windows on each side of the gunner so that he may 
pick up a target approaching from the left or right and also a large 
round window between the guns to allow him to sight and follow his 
target. 

When the guns are at 0° elevation the turret door is outside the body 
of the plane. When the guns are brought to 90° elevation the ball 
swings so that door moves up into the inside of the plane. Since the 
guns would be smashed against the ground if they were at 90° when 
the plane took off or landed, the gunner must wait until the plane is 
in flight before he can swing the turret and turret door around in 



100 


order to get in. Thus, in take-offs and landings the gunner should 
never be in the turret, and the guns must be positioned at 0° elevation. 
Before getting in, the gunner should make certain that all switches are 
off and that the manual clutches are engaged. If he neglects to do 
this the turret may creep as he tries to enter and he may suffer serious 
injury. 

The gunner aims through his sight on out between his feet and his 
two guns. He introduces range into his sight and frames his target 
by means of a foot pedal which he manipulates with his left heel. 

SPERRY.LOWER REMOTE TURRET 

General. 

1. Use .—Mounted just to the rear of the radio room in the bottom 
of the fuselage of B-l7’s to protect their lower hemispheres. 

2. Guns .—Two .50-caliber M-2 machine guns parallel to each other. 

3. Power .—Two hydraulic units, one for azimuth and one for eleva¬ 
tion, and an electrical motor for driving them. 

4. Movement .—Turret turns a complete circle, but guns are limited 
in movement from 0°-85°. 

5. Control .—The gunner manipulates the turret by remote control. 
He operates manually a sighting station, which is 8 feet to the rear 
of the turret and which consists of a ring, a cradle, and a mount for 
the sight. The movement of the sighting station transmitted to the 
turret by means of a sighting station transmitter. 

6. Speed .—Same as upper local. 

7. Ammunition .—Same as upper local. 

8. Charging .—Guns are charged by means of Bendix hydraulic gun 
chargers. 

9. Sight .—Sperry automatic computing sight K-5. See “Special 
features” section of Sperry upper description. 

Operation. 

Except for its remote control the turret operates in a manner similar 
to the Sperry upper local. 

Special features. 

In general the special features and disadvantages are those of the 
upper local. The lower remote turret has worked well but has not 
proved as successful as the Sperry ball turret, which is rapidly re¬ 
placing it, due mainly to the reduced field of vision of its remote 
sighting mechanism. 

BENDIX LOWER TURRET 

General. 

1. Mounted in belly of B-24’s and B-25’s. Field of fire covers 
all approaches to ship’s bottom and a few degrees of the aft area above 
the horizontal. 

2. Guns .—Two .50-caliber M-2 machine guns. 


101 




3. Power .—Electrically driven power turret. Controlled by safety 
power switch on back of control handle. 

4. Movement. —360° in azimuth; 88°-180° in elevation; i. e., from a 
few degrees above horizontal to straight down. 

5. Hand control .—A single control handle operated with the right 
hand controls direction and speed of gun movement. 

6. 8feed .—per sec. to 40° per sec. in azimuth and elevation. 

7. Ammunition .—The ammunition is held in containers which are 
fixed to the tun^et housing and which feed ammunition to the guns over 
rollers. 

8. Charging .—A charger valve is located on the left side near the 
steady grip. Guns are charged ready for combat when the charger 




















102 


control valve is depressed with knob rotated clockwise against the stop. 
Guns bolts are held back in safety position when the valve is depressed 
with the valve rotated counterclockwise against the stop. 

9. Sight .—Bausch & Lomb periscopic sight used exclusively in Ben- 
dix turrets. No magnification (target viewed as with naked eye). 
40° cone comprises field of view. No computing or compensating 
devices for ballistics. Some of the turrets’ sights have installed a 
mechanism which automatically makes corrections for a fixed range of 
1,800 feet for indicated air speed, ballistics, velocity, and direction of 
target. At present, however, this automatic sight compensator is 
not perfected. 

Retraction .—To provide ground clearance for landing and to reduce 
drag, the turret may be power retracted when it is not in use. The 
housing then fits into the turret well, and the guns rest within slots in 
the fuselage belly. In this position the firing circuit and the circuit 
controlling the movement of guns in elevation are automatically dis¬ 
connected, thereby preventing damage to the ship. 

To lower the turret from its retracted position, the gunner grasps 
the control handle with his right hand and depresses the safety power 
switch on the back of the handle. Current is thus supplied to various 
parts of the turret. He then rotates the control handle counterclock- 
Avise about its vertical axis, causing the center column and turret to 
be loAvered. 

To retract the turret into its stowed position, the gunner places his 
guns at the proper elevation so that they will nest into the bottom of 
the ship, depresses the retract lever at the top of the center column, and 
rotates the control handle clockwise about its vertical axis. 

Operation. 

1. Affamtus .— 

(a) Control handle .—Mounted on control box to right of oper¬ 
ator ; houses main power switch and trigger firing switch. 

{1) Steady Mounted to left of operator, for his left 

hand; houses interphone microphone switch. 

{c) Charging handle .to right of steady grip; oper¬ 
ated by heel of left hand. 

{d) Sight .—Extends through the axis of the turret; eye cushion 
furnishes protection to eye. 

(e) Index lever .—Located on center colmnn just below eye 
cushion. 

2. Extension to combat position .— 

{a) Before extension, make certain index plate spring holds 
retract bolt in center column. 


103 


(h) Turn control handle counterclockwise about its vertical 
axis. For slow speed turn onl}^ a few degrees. For maximum 
speed turn all the way. 

(c) Grasp control handle firmly with right hand to depress 
main power switch. 

(d) When turret is almost all the way down, use slow speed 
until indexing key automatically engages and guns start to rotate 
clockwise. 

(e) Do not touch firing trigger switch on front of control han¬ 
dle, since the guns can fire as soon as extended position is reached. 

(/) Charge guns. 

3. Combat maneuvers .— 

(a) Take kneeling position with right hand on control handle, 
left hand on steady grip, and eye on sight cushion. 

(Z)) To move guns in azimuth rotate control handle about its 
vertical axis. Guns will move in same direction as handle, and 
the speed will vary according to the degree of rotation of the 
handle from its uormal position. 

{c) To swing guns in elevation, move control handle up or 
down. 

{d) Look through sight, and train guns on target by means 
of control handle. 

{e) Judge lead and fire by depressing trigger firing switch 
on control handle. 

(/) To make contact with microphone, depress button- on end 
of steady grip with thumb. 

4. Retraction to stowed position.-^ 

(a) Slowly rotate turret clockwise with guns 10° to 15° below 
horizontal. As guns approach 90° before aft position depress 
retract lever. 

(d) When turret stops, slowly raise guns in elevation. After 
guns enter elevation index zone, turret will turn clockwise again. 
Stop guns from moving in elevation. Let turret slowly rotate 
clockwise until turret index is at aft position and retraction 
begins. 

(c) Caution. —Keep control handle set for slow azimuth speed 
while guns move in elevation. 

[d) After turret has retracted about 1 inch, release index 
lever and continue retraction at any desired speed to stowed 
position. 

{e) Kelease grip on control handle to cut off power. 

5. Emergency retraction.—li the power supply should fail when 
the turret is in extended position, it is possible to retract the turret 
by manual control. 


104 


(a) Loosen and remove windshield. 

(b) Remove manual control crank from retainer under chest- 
support assembly and place it on extension shaft, which is at¬ 
tached to the hanger-arm assembly. Press shaft down until it 
engages with elevation speed reducer. Looking down, turn crank 
clockwise to move guns slowly in elevation. Turn crank until 
guns are in a stowing position. 

(c) Remove crank from elevation speed reducer and install it 
on end of shifter shaft, which is located in azimuth gear housing. 

(d) Pull shifter shaft assembly out about one-half inch from 
azimuth gear housing. Rotate guns clockwise, and hold retract 
lever depressed as guns swing to aft position until turret indexes 
and starts to retract. Now release to a fully retracted position. 

Notk.— This procedure throws the azimuth compensator off so that it 
must be retimed. 

The single control handle and its action has proven very un¬ 
satisfactory in its early models. A new type double control han¬ 
dle is being developed and a new control system which should 
remedy this situation. 

BENDIX UPPER TURRET 

The Bendix upper turret except for the retraction features is almost 
identical in construction and operation with the Bendix lower turret. 
It is mounted in the B-25 to protect the plane’s upper hemisphere. 
The field of fire is controlled by cams and switches so that the guns 
cannot fire into any part of the ship, but the turret itself rotates 360° 
in azimuth and 0°-90° in elevation. 

This turret has had the same control difficulties as the Bendix lower 
remote turret. 

CONSOLIDATED TAIL TURRET 

General. 

1. Mounted in extreme tail of B-24’s between the two vertical 
stabilizers to protect against rear approaches. 

2. Guns .—Two .50-caliber M-2 machine guns. 

3. Power—Hydraulic.—\\\^ single Vickers hydraulic unit, which 
moves the turret in both azimuth and elevation, gets its pressure from 
a gear pump driven by a constant-speed electric motor. 

4. Movement.—\\\ azimuth the turret moves 70° to either side of 
directly aft. In elevation the guns move 85° above horizontal and 
40° below horizontal. During the last 10° below the horizontal, how¬ 
ever, the gunner’s line of sight is obliterated by the lower armor 
plating. 


105 



Figure 102. 


5. Hand control. —Control handles, shaped like a letter “A” and 
operated like bicycle handle bars, serve to move the turret left and 
l ight and the guns up and down. 

6. Speed. —The farther and faster the control handles are moved 
from their neutral position the faster guns and turret move. 

7. Ammunition. —400 rounds in left gun can, 400 in right gun can. 
Monorail system enables gunner to refill cans. 

8. Charging. —Eight gun is charged by pulling back right monorail 
charger handle with left hand; left gun by pulling back left charger 
handle with right hand. 

9. Sight. — NS A. —See Martin turret; also chapter on Optical Sights, 
Mk. 7. 

Operation. 

1. Enter fuselage, and before getting into turret turn on master 
switch in the tail. 

2. Open doors, and using hand grips on inside above turret door, 
swing in. 

3. Turn on main shut-off valve between legs (three or four turns). 

4. Charge guns. (See above.) 

5. Turn on sight and adjust rheostat located on switch panel to right 
side of turret. 

6. Turn on gun safety switch, also on panel, thereby closing circuit 
to guns and causing green signal to light. 







106 


7. Turn on receptacle switch to adjust heat for fly-suit. 

8. Grasp control handles, and with them move turret in azimuth or 
elevation to line up target in sight. 

9. To fire, press trigger switch with right index finger. 

Special features. 

1. Three warning lights on the switch panel indicate whether the 
guns are operating correctly, or if not, which gun has a stoppage. 
One of the lights is green, the other two red. When the guns are 
operating correctly, the gi*een light is on. When one of the guns 
ceases to fire, the green light goes out, and the red light corresponding 
to the dead gun lights up. 

2. There are no dead man switches. 

3. The master turret control valve, operated by the control handles, 
permits the turret to move in azimuth and the guns to move in ele¬ 
vation, both at the same time. But the turret cannot move as fast 
this way as it can in elevation or azimuth singly. 

4. A monorail running along the top of the plane’s fuselage carries 
1,600 rounds of spare, belted ammunition. When the gunner gets low 
in ammunition, he opens his doors and pulls on a cord attached to the 
belted ammunition. The belt then slides down the rail to the door, 
where he lays it in folds in his own ammunition cans. 



Section XV. RECOGNITION 


Recognition is concerned with the accurate identification of aircraft. 
Although the first step is distinguishing with certainty between 
friendly and hostile planes, this alone is not enough. As will be seen 
later, in combat it is often vitally necessary not only to recognize the 
type of plane and the particular model, but also to know certain 
basic facts concerning its performance, armament, and vital spots. 
The continual growth in the number and variety of planes in military 
use makes what might at first seem a simple problem rather complex 
and makes it absolutely necessary that anyone expecting to see combat 
be thoroughly acquainted with the most common planes before he goes 
to the fleet. 

Need for recognition. 

The importance of swift and accurate recognition can hardly be 
overemphasized. The annals of the war are already filled with tragic 
stories of planes and pilots shot down because their own defenses mis¬ 
took them for enemies. For us, it is amusing to know that a squadron 
of Italian planes bombed and sank several of their own ships, but it is 
far from amusing when we hear of the tables turned, when we hear, 
for example, of the sentry in Oregon who mistook two of his own 
planes for Japs and ordered fatal antiaircraft fire against them. 
Perhaps the recognition story Americans least like to hear is that of 
December 7, when the tragedy of Pearl Harbor was climaxed at dusk 
by our own gunners’ shooting down numbers of SBD’s and F4F’s. 
It is hardly necessary to multiply such examples in order to illustrate 
the importance of distinguishing with certainty between allied and 
enemy planes. 

Type of acquaintance needed. 

But such a simple distinction, although in some cases it may be 
enough for civilian spotters, is far from enough for the gunner, whose 
job is to protect his own plane and men, and destroy every possible 
enemy. He must not only recognize a plane instantly and certainly, 
he must know, and know immediately, certain facts about it if he is 
going to have a fighting chance to shoot it down. These facts can be 
divided into three basic groups. 

1. DimeTisions .—The effectiveness of the gunner’s fire depends en¬ 
tirely upon the accuracy with which he estimates the range of the 

( 107 ) 


493667°—43- 


-8 


108 


enemy. Upon this range estimation depends the whole method of 
apparent speed sighting. And in order to estimate range, the gunner 
must know immediately and accurately the span and length of the 
plane at which he is going to fire. If he does not know these facts, 
his sights and his guns are useless to him. And obviously, before he 
can know them, he must recognize the plane. 

2. Vital spots .—To destroy an enemy plane, it is not enough merely 
to score a hit. Stories are common of planes that return to their bases 
with wings, fuselage, and tail shot full of holes. The gunner must 
know where the plane is most vulnerable, what are its weakest spots. 
The more he can know about the armor of a plane, the location and 
vulnerability of its motors, fuel tanks, and pilots, the more chance he 
has of sending it down in flames and himself returning alive. 

3. Armament and performmice .—If a gunner knows accurately the 
dimensions and the most vital spots of an enemy plane, and uses his 
information efficiently, he will be a good gunner. But if he can train 
himself further to know the armament and the performance char¬ 
acteristics of the enemy, he can improve his score even more. If he 
knows where the guns of the enemy are located, what kind of guns 
they are, what their field of fire is, how fast the plane is, and how fast 
it can climb or dive, he can foresee more certainly what kind of attack 
he may have to defend himself against. In short, once the gunner has 
mastered the basic facts necessary for performing his job, the more 
he can know in addition, the better gunner he will be. 

Last, and perhaps most significant of all, is the need for what 
may be called trigger recognition. The high 'speeds and great 
maneuverability of modern planes leave the gunner only a few 
seconds to recognize the enemy, sight his guns, and fire. If we 
consider the extreme case of two planes approaching each other, 
each flying at 300 knots, they will be closing at a rate of 600 knots. 
If they are 5 miles apart when they sight each other, they will 
pass in 30 seconds. In the perhaps more common case for the 
free gunner in which an enemy attacks from the rear at 300 knots 
and the attacked plane is flying about 250 knots, the gunner must 
recognize the plane almost instantly if he is going to estimate his 
range and speed, and to fire effectively before the attack is com¬ 
pleted. It is not sufficient, therefore, for a gunner to be able to 
figure out the identity of a plane in a minute or two; he must 
know it certainly and immediately. This ability can come only with 
careful, constant study and practice. 

Training devices. 

It must be apparent that the ideal situation would be that in which 
the recognition student could see and examine the actual planes which 
he was studying. But it is equally apparent that this is impossible. 


109 


To compensate for this, use should be made of all possible visual aids. 
At first thought, pictures would seem among the most useful of such 
aids. But in practice, useful as they may prove, pictures offer certain 
difficulties. A plane is seldom seen in flight in the same detail in 
which a picture shows it; hence it is easy for the student to acquire 
the habit of recognizing a plane in a picture by some characteristic 
which will never actually be visible to him under combat conditions. 
Silhouettes, however, meet this objection and furnish one of the sim¬ 
plest and most practicable means for practice. In conjunction with 
such silhouettes, scale models can and should be used. Such models, 
hung where they can be seen continually in actual flight positions, 
or placed where their structural details may be examined closely, 
prove an invaluable aid to the student, and all possible use should be 
made of them. The student should be aware, however, that the scale 
model takes time to make and requires accurate plans if it is to be 
useful. Hence models of the latest planes are much more difficult to 
make available than silhouettes. 

Until very recently the Weft system for recognizing aircraft was 
used by the Navy. Now, however, this system is being replaced by the 
Renshaw method of identification. Literature describing the Ren- 
shaw method and the instruction of it can be obtained from the Train¬ 
ing Division, Bureau of Aeronautics, Washington, D. C. 


Section XVI. RANGE SAFETY PRECAUTIONS 


Only the rigorous enforcement of safety rules makes it safe to ap¬ 
proach the gunnery range when scores of beginners are practicing 
with murderous weapons. The slightest carelessness due to momentary 
confusion may cost a life. 

A concise statement of the safety precautions governing each range 
should be carefully formulated. No person should be permitted to 
practice on the range before he has read and understood the state¬ 
ment, and declared in writing that he has done so. Each safety pre¬ 
caution has been formulated in the attempt to prevent the repetition 
of some casualty. 

{a) Always assume that a gun is loaded until you know it is not. 
The only way you can know a gun is not loaded is to remove the maga¬ 
zine, open the bolt, and examine the chamber; or to cock it and pull the 
trigger twice with the magazine oif. This second method is only to be 
used when the gun is mounted on the firing stand and pointed toward 
the butts. IF A GUN HAS BEEN OUT OF YOUR SIGHT SINCE 
YOU DETERMINED IT UNLOADED, IT MUST AGAIN BE 
TREATED AS A LOADED GUN. 

(Z)) No gun will be loaded at any time except when installed in 
firing stand and pointed toward the butts. 

(c) The safety will be kept on all guns, whether loaded or not, 
except when actually firing. 

{d) No gun will be removed from any stand until the magazine 
has been removed, the gun determined to be unloaded, and the safety 
placed on. 

{e) No firing will be done from any stand during the absence of 
the instructor. 

(/) Each stand will fire only at the targets directly in front of the 
stand (fixed targets only). 

{g) The front edges of the firing stands make a deadline which 
shall not be crossed during firing. 

(A) No gunner will leave his stand until his gun has been prop¬ 
erly unloaded, placed on safety, and inspected })y an instructor. 

{i) Guns on firing stands will always be pointed toward the butts 
or at the ground directly in front of the stand. 

(^) Stops are installed on all firing stands to prevent firing over 
the top of the butts. However, each gunner is personally responsible 

( 110 ) 


Ill 


for checking up to see that he cannot and does not fire over the top of 
the butts. 

{k) Breaking down of live ammunition is prohibited. 

{1) At the command “CEASE FIRING” or when the cease-firing 
gong is sounded the safety will be placed on all guns and the guns 
will be pointed at the ground directly in front of all the firing stands 
with the bolts open and the covers raised. 

(m) The command “COMMENCE FIRING” will be passed by 
word of mouth. 

{n) In case of a misfire or jam on a machine, pistol, or shotgun, 
keep the gun pointed clear until the trouble is remedied. 

(o) While at the pistol butts do not turn away from the butts 
while you have a pistol in your hand. 

{p) In the case of misfires, the cartridge shall be placed in the box 
provided for that purpose. 

Note that items {i) and (1) do not refer to the pistol and shotgun. 
These weapons are pointed up at an angle of 45°. 

When a misfire occurs with a rifle or shotgun (item m), wait for 10 
seconds before opening, lest there be a hangfire. 

Before leaving the pistol stand, 

(1) Remove the clip, 

(2) Pull back the slide, and 

(3) Look in the breech end to make sure the barrel is empty. 

At the machine-gun stand, keep the cover up when not firing. 

After firing, 

(1) Remove the cartridges, 

(2) Pull back the bolt two or three times, 

(3) Push the safety from right to left. 


Section XVII. GLOSSARY 

Angle of Deflection.— The angle between the sight axis and the 
line of sight. 

Apparent Speed. —The speed at which the target appears to be 
moving along the line of apparent motion. This motion is in a plane 
perpendicular to the sight axis. 

Apparent Wind Deflection. —The apparent deflection of the pro¬ 
jectile caused by the loss of speed due to air resistance along its 
trajectory. 

Approach Angle. —The angle between the line of sight and the 
target’s line of flight—(eye to tail to nose). 

Axis of the Bore. —The longitudinal axis of the barrel of the gun. 

Azimuth—(of the Gun). —The horizontal angle, expressed in 
degrees and measured clockwise, between a vertical plane through 
the line of flight and a vertical plane through the axis of the bore of 
the gun. 

Boresighting. —The adjustment of the sight axis and the axis of the 
bore according to a predetermined condition. They may, for instance, 
be made to converge at a given range. 

Caliber. —Caliber is the diameter of the bore of a gun expressed in 
inches. It is usually used only where this diameter is less than an 
inch. The same word is also used as a measure of length, that is 

... length of bore in inches 

caliber = — 7 - 7 --^^— 

diameter 01 bore in inches 

Cone of Fire. —The portion of space which contains the trajec¬ 
tories of all bullets fired from a gun fixed in azimuth and elevation. 

Deflection — Gunner’s. —See Lag. 

Deflection, Target. —See Lead. 

Drift. —The horizontal movement of the bullet to the right of the 
axis of the bore caused by the greater air pressure on the underside. 
The clockwise rotation of the bullet makes it roll to the right. 

Exterior Ballistics.— The study of the motion of the bullet after 
it leaves the muzzle of the gun. 

Firing Angle.— The angle betwen the axis of the bore and the line 
of flight of the gunner’s aircraft. 

Flexible Gun. — See Gun, Free. 

( 112 ) 



113 


Gravity Drop. —The vertical drop of the projectile from the axis 
of the bore due to the pull of gravity. It increases with range. 

Gun, Fixed.—A gun mounted rigidly in an aircraft in such a way 
that the whole aircraft has to be maneuvered to aim the gun. 

Gun, Free.— A gun so mounted that it can be aimed independently 
of the position and motion of the plane. 

Harmonization. —The adjustment of two or more guns so that their 
bullets form a single pattern. One sight controls all the guns. 

Immediate Action. —An automatic routine performed by the gunner 
to eliminate a stoppage quickly. 

Interior Ballistics. —The study of the motion of a projectile before 
it leaves the muzzle of a gun. 

Jump, Windage. —Deflection of the projectile from the axis of the 
bore caused by the projectile’s rolling across the “relative wind” on 
the forward side of the projectile. 

Lag. —The correction a gunner makes due to the motion of his own 
plane. It is always along a line parallel to that motion, but in the 
opposite direction. (Also called gunner’s deflection.) 

Lead. —With a moving target, the amount the gunner shoots ahead 
of the target in order to allow for the motion of the target. It is 
always along the line of flight of the target. (Also called target 
deflection.) 

Line of Apparent Motion. —In apparent speed sighting, the line 
along which the target appears to move. It is always in a plane 
perpendicular to the sight axis. 

Line of Sight. —Line from the eye to the target. In a no-deflection 
shot would be same as axis of sight. 

Malfunction (of a machine gun).—Improper operation of a gun 
due to a faulty, misplaced, or missing part. 

Mil. —An angle of 1 mil is the angle determined by the arc of 
length 1 foot in a circle whose radius is 1,000 feet. 6,400 mils=360°. 
(Called mil because the arc is 1/1000 of the radius.) 

Eeflector Sight. —^An electrical sight which contains an optical sys¬ 
tem such that the image of the bead and rings or reference lines are 
seen through a glass as projected on to the target. 

Eelative Speed. —Speed at which plane is moving relative to an 
observer. Differs from apparent speed in that speed toward or away 
from observer is included. 

Eelative Wind. With respect to a projectile the wind that is the 
resultant of the wind due to the motion of the aircraft and the wind 
due to the motion of the projectile. 

Sight Axis.—A line from the gunner’s eye through peep and bead, 
projected into space. 


114 


Sight Base.— Distance from gunner’s eye along axis sight to 
ring. 

Stoppage. —Of a machine gun, an accidental cessation of fire due to 
a malfunction or faulty ammunition. As distinguished from a pro¬ 
longed stoppage, a temporary stoppage is one which can be eliminated 
in flight by the routine procedure called immediate action. 

Telescopic Sight. —A nonelectrical optical sight instrument con¬ 
taining a convex or convex-concave lens and which in aerial machine 
gunnery usually has no power of magnification. It performs the 
same function as the reflector sight but is not illuminated and allows 
the gunner almost no head movement. 

Time of Flight. —The time required for the bullet to travel from 
gun to target. 

Trail. —Distance of the trajectory from the axis of the bore. 
Sometimes divided into horizontal and vertical components. 

Trajectory.— The actual path of the bullet. 

Vector. A line segment whose position indicates direction and 
whose length is scaled to indicate magnitude. 

Velocity, Instrumental. —The velocity of a bullet 78 feet from the 
muzzle. (At one time 78 feet was the shortest distance from the 
muzzle at which the velocity could be measured accurately.) 

Velocity, Muzzle.— Speed of a bullet when it leaves the muzzle. 
Also called initial velocity. 

Yaw. —The angle between the longitudinal axis of the projectile 
and the tangent to the trajectory at any point. (If the trajectory be 
considered a straight line it is the angle of the axis with the trajectory.) 

Zenith Distance (of a gun).—The angular distance, expressed in 
degrees, from the zenith to the axis of the bore; the angle the axis of 
the bore makes with the vertical. 


Section XVIII. SYNTHETIC DEVICES 

A great deal of the initial training of the free gunner cannot be 
carried on in the air. In many cases, more can be accomplished on 
the ground. In all cases, the training should be as realistic as con¬ 
ditions will permit. There are a number of synthetic devices which 
have been or are being developed to teach the student the solution of 



Figure 103.—Range and speed estimation sighting bar. 

the problems in the air. The most important of these are illustrated 
and described briefly in the following pages. 

The range and speed estimation sighting bar manufactured at the 
Naval Air Station, Pensacola, is a 35-mil ring sight mounted at the 
end of a 20-inch bar. This can easily be used by students in esti¬ 
mating range and speed. 

RANGE ESTIMATION RANGE 

The range estimation range was developed at the Naval Air Sta¬ 
tion, Pensacola. This range drills the students in recognition and 
range estimation with %o or Y^q scaled models at correspondingly 
scaled ranges of 1,000, 2,000, 1,500, and 500 feet. This range is very 
effective in training groups. (Fig. 104, 105.) 

SPEED ESTIMATION RANGE 

The speed estimation range was developed by the Synthetic Train¬ 
ing Department, Naval Air Station, Pensacola. It has a variable 
speed motor to simulate speeds of 0-100 knots. Students are drilled 
with %e scale targets and speeds, and ranges representing 1,000 and 
2,000 feet. (Fig. 106,107.) 

BB AIR MACHINE GUN 

The BB air machine gun is manufactured by the McGlashan Air 
Machine Gun Corporation, Long Beach, Calif., for the Special De¬ 
vices Section of the Bureau of Aeronautics. This device is used 
for training gunners in gun handling and follow through. %o scaled 
targets are moved at scaled ranges and speeds. Lead is corrected 

( 115 ) 












































































































































p 


/ 


I 


*. ► 


I 

117 


! 











MiW 


m^mn 










Figure 105 . 


i • 


































118 


'i 



t 




Fiouke 106 . 

I 

i 

\ 

1 

% 

1 

I 

• i 

•ii 

V 

• 1 


.V 

•j 















































119 

V 




\ 




FIGURE 107 










120 



Figure 108 



































121 









■V ' 

< J( ■•••} 

'rs^ 



■« :X:-; 


■x 




f'' 


•+'iS-V 



Figure 109 























































































s . 



» JPT 





ft * 


> 



Figuue 110. 





































































































































































7 - 


123 

by offsetting the sights frorn the direction of lead. (This is necessary 
because of the high muzzle velocity—450 feet per second—which is 
much faster than a Voq scale velocity.) 

SHOULDER-HELD SHOTGUN WITH RING SIGHT 

The shoulder-held shotgun with ring sight (sight produced at 
Naval Air Station. Pensacola) enables the gunner to lead clay pigeons 



Figure 111.—Remington IModel 31 (pump). 


by variable amounts of the ring. This gun is very valuable i)repara- 
tion for the sjiade-grip held shotgun and the moving base shotgun. 
The ring is constructed with a 20-inch sight base and radius of 1 



inch. This allows a lead of 3 feet at GO feet (approximate trap and 
sheet ranges). 

493067°—43-9 















124 


SPADE GRIP HELD SHOTGUN ON MACHINE GUN INIOUNT 

The Spade-Grip-helcl shotgun on a machine-gun mount was pro¬ 
duced by the Special Devices Section of the Bureau of Aeronautics 
(Code No. 3A7). This device enables the gunner to lead clay pigeons 
by variable amounts in relation to his ring, and allows the gunner 



Figure 113.—Spade grip held shotgun. 


to become accustomed to firing with a weapon mounted in a machine 
gun mount adapter. It is suggested that this ring be replaced by 
one similar to the one used on the shoulder held shotgun. (See p. 123.) 

3A2 TRAINER 

The 3A2 Trainer is manufactured by the Jam Handy Corporation 
for the Special Devices Section of the Bureau of Aeronautics. This 



is an advanced trainer to be used by students after they have been 
drilled in range estimation, speed estimation, and apparent speed 



Figure 114.-—3A2 Trainer. 


sighting. One projector projects realistic attacks, while the other 
indicates the proper leads. 


JONES TRAINER 

No. 5C Trainer (Jones Trainer) is maniifactiired by J. W. Jones 
for the S])ecial Devices Section of the Bureau of Aeronautics. This 
trainer is used to train gunners in range estimation. This device 
must be modified by local stations to correct the readings to feet. It 
is not well adapted to group instruction. 












126 





















































127 


AIRCRAFT RECOGNITION BOX 


Tlie Aircraft Recognition Box was produced by the Synthetic Train¬ 
ing De])artnient at the NaA’al Air Station, Pensacola. 



Figure 116. 














128 





Figuke 118 . 




J 

i 


A 
















































129 



SHIP RECOGNITION BOX 

The Ship Recognition Box was manufactured by the Special De¬ 
vices Section of the Bureau of Aeronautics and modified at the Naval 
Air Station, Pensacola. 


Figure 119. 


MOVING BASE RANGE 

The moving base range was designed by the Gunnery Depm'tment 
at the Naval Air Station, Pensacola. The cars were manufactured 
by Fairmont Motors Corporation, Fairmont, Minn. This device 
consists of a gasoline-powered car on oval track. The gunner is 































130 



Figure 120. —(’ar for shooter. 















131 


taught to hit a moving target from a moving base using a shoulder- 
held shotgun with a ring sight. 

HIGH-SPEED TARGET RANGE (FULL SCALE) 


The high-S])eed target range was developed by the United States 
Army Free Gunnery School, Las Vegas, Nev. Tlie car is manufac- 



Figuke 122.—Target on car. 


tured by Fairmont Motors Corporation, Fairmont, Minn. This high¬ 
speed target traveling at speeds up to 60 knots enables the gunner to 
fire service machine guns, either hand held or in turrets, at ranges 
up to 2,000 feet. 












































CUTAWAY MACHINE GUN 


Cutaway Browning Aircraft Machine Gun, caliber .30 M2, used in 
the stripping room at the Naval Air Station, Pensacola. This power- 


! 



Figure 125. 


driven model is most effective in demonstrating the operations of the 
gun. 


U. S. GOVERNMENT PRINTING OFFICE: 1943 























\-'. '■ r 

',;• 4 ^^*** • 

U.,V;i.’ 

■ ■*^'^’ ' V'* 

"1 ** V’' * ' : ■ . 

1 j. 

1. ^ 

* •■ ■ ^ ► . • 

. W ' 

J 4 4 ♦ ^ 

: ' ■» 


..r't: 


'\ . ■- ->fv "^.v ' ' '* 




i;^.v* \. ~V^±' -kv A.-’, *■ ’? •■'■.*’;.r^ ■ •;"V.9’>'' V'*'•' ?-i 

* ♦Vi ; .J ^ - ^1 ■ '• •>*' JTJ^ *••-JTlcff- <3 

■* . V * • ' ^ 1 ♦ ■ ■ ▼ ■ ? A-kj ' 4 ^ ^ * • « ^ * • ’ J , • 7i^T Ta “ t 









'■.■..A- ' 

V . : 






: , r. ■ ‘ , ‘ v%..; 

■ ' vv ^ * ' ' 1|\ ^ I .'l 

• >:<.*> ;<^7 -, 

f- ’t* • ,. ■• . •* 

§v,) ^ 


; '( ' * T'/ .»'*.* 

I 0 , ' ^ •' ' 

1 . f ' • .. V. • •• . 



>jrr.* ^ \'H • 1 ' , { 

•' ■<^ .■ . •- ,• ..... '1 < -,, -, 

■'• ■ V'*'/'■'•■:'.K ' 7'‘'- . V'7' '• 

" . ^ :,' ■■■ ■■ ' 

• ■;■• •• ^ ' ■■• • -v- . 

. <. ,4 ■ -,. ■''f T- •' -• 

' ’■•• •* .'f"- ■''- •'. 4 i--’ . '•• : ■ ' • • .'••■■• 


•-x.' ■ .♦ 




-. y*".'' • 

■■'r. . 

• . X -. I 




^ ^ , H’ I '?/ 


, -.V - V'■•*?. 


•:»./ '.‘■■Ti -f:' ■■. -' . 

' - • -a. ' r . . 




'i. •' 




•X- 


I 


.'j' 

»i'" 


1^ ., 

> » 


• ■ ^ -1 ••-,'• 
'■'7, 

' ;i. ■'*’ ''' i ,'« i-' .■-' '' ' 

•- -- l* .. - a 




. . • - "S-' 

. ■' ■■viM# 

■ -wsM 



.!.♦ 


. • k' 


■M. ^." • 




. .v 


A' i 


:• - v-’'' 


7 » ‘ »• 


'V 


A' 

f' *.' 

^ <<• • ■. 




•' *'' • > j • —'. ^ 4 ' » ' . ^ ‘ ■ • , 

n;.. ,■ , ‘.. • .•;• 'I' ':■ ';. ■•■^v . r.,.- ., 

!■ ', -i,: •' ■’.< ''' . : ■;.' ' ■ ">- i. ■ •-,■•? 

u ; ■ ' '' -' ■' '■ U7.'.' ■ ■ 

■'■■.f: -:*■.■•'^ ■•^^v;v.- . ■: ' ■ •: 

'.> .■■ - ■ **’ 

/■ 


f :• ■■-?••-' 

fV, 


• 7'--7' 






- t .. - (> 

jv.' '^. .:-- 


^ (’ 


■.• •'. . ,..• ;^-' > . 4y ■•■»-.. . 

. ' '{ ' \~ .f ■■ '- I 

•; ■•'/' •' - /. •■ >•.' 


■ ■ 

t .'. » 




^'-7"^'/‘r--’:. ^ 'r '- ■ ■ •'■ 

,'■-•• :-.7v. ^>-•••^■ .'^ ■• ^^,‘>• 7v'/ : v'• 

• - , "'■■ ■ '1 • •• - '' ■' i‘' V .. 

■-.. ■■ ■:, 7' v!:77-' • 7-7 

7 ■ ■■v7777.' l-. I/:.'- .v7'':'7r“ 7 ■ V ^ v:.;, 

■V.' ■■■-• :\y^^ 7 ;■: ■,- ■ , 7-^-■■ ■ 

•• . '•“■■!. .'>- 'I*' '’f- I •'■*'. -fi ,-i,'.• •, S’* ' • 

■^ . ■ .j , ‘ ' ^ - . • . - , ■ , - .*• • f . i-a • ■* , 


> k 




-■v , , • --v. 


< 

• V 






V, i 

- ,v. ..'. -v ^:'v^',^: ■,,> .,,: 

■ 77 .7'.;; v"77:77.".:': '7;.'■■■ ■ 

... . i • » '' '* •.’■•"■'.,• ' ‘ ' s'* ' X.' ‘ . 

' ^ ,.' . I H i' ‘: - .> ^ * j **>,. • ' '' 

■ *. - •*. • * -. •• 


-■’i •.7^ -* 

/1 


m 

V 

/ 

7i:- 


-’•^7^' 


■■ ■ ■.. .7-,. - ■ ''V '7 ■; ■■.■' ) ' 7'; ■>.. 




»• 


■$>. 


, *“1 


' c. 


• ‘.v 




A. 

' t' 




r ' 






'iS ' 




■'¥7 


•, •' ^ 


1 . >. 

s'* 

- >■’ '= 




•v 


/ - ■■ ' ^ 

• -.K. • ■ \%i 

‘ ‘ >■* X J 

'■.r. kvA' 

- 

s'- l> 4^ 


• , *1. 

X ' 


% 


7-'. 

.si' • »■ 


' • -A ■ . 

X ; 


-> .. > 


•a 

^ '' 


•' a,' 

■V - 


'i:-' 


' t 


. ' *rC-- 


«* . • 

., I 

, ■» • 


••J, rt 

. < ■'' ■, 




• 5 


, v;;.A ■ 


'■fi 


.'* ■- 

' ■'> ' 1 ? - 

'-■ c V 

*' •// *'• 

.»-• 


■,■7^" 7- V' 


■I' -. •»• 


/ - 


' f ^ •^p 

;.X‘- ,» '.•■•«■ .' 

,;/ i'it; .. . '. -',. . ■*»;.'-•• .’V7 • .’ •'■ '•' 

.. V. - *, ,' '.. '••'■ -i. ‘.V; ■• 

■' •• ■ '.7" • ■'• 


. 



















































